In my opinion, high elves should not be an allied race. A few reasons why:
1. Blood elves are an integral part of the Horde and form a part of the Horde's identity (whether you like it or not). Imo, playable high elves would claim a part of that Horde identity... Void Elves have already done that to a degree, but adding HE would further this. Although somewhat culturally different, the HE would look and play like a BE nonetheless. I have seen ideas thrown around as to how you could make changes to the HE model to differentiate it from BE, but without changing the BE skeleton (say to HE skeleton) you are going to end up with something that still pretty much looks and feels like a BE. Changing the skeleton to human or NE could be a solution, but I get the impression that many HE supporters would not like that.
2. Further to point (1), some HE supporters use the excuse that because VE are now alliance, they're now entitled to have altered BE models (because they're now alliance models too). I understand the direction this point of view is coming from, but I somewhat disagree. Regardless that VE are now alliance, the BE model IS and ALWAYS has been a Horde model in WoW. As a predominantly Horde player, I would feel cheated that one of my factions models has been given to the alliance (with more customization options... between VE and HE). I understand Nightborne were allied to the Horde and were given different customization options, but the same happened with BE -> VE. Simply stated, I would feel cheated if the alliance were given more options for the BE model than Horde, given that its a Horde model through and through. Also, some HE supporters argue that BE should never have joined the Horde, and rather HE should have been a playable alliance faction; this is an invalid point, because regardless of your views of "how the story should have been", that's how the story has been written and that is the current and correct Lore.
3. Popularity of HE. A worry for some is that adding HE might cause a faction imbalance in favor of the Alliance. Currently, the factions seem to have a relatively balanced population (albeit somewhat scaled higher toward the Horde... less than 5% difference). Some HE supporters claim that there is a lot of support/demand for playable HE. If this is true, then there is a risk (could be a low or high risk, but a risk nonetheless) of population imbalance. VE are by far the most popular allied race (for obvious reasons... ie. BE model on alliance), so assuming that HE would far more popular than even VE (given the large amounts of comments claiming VE were an A$$ pull and that it should have been HE as an AR) we have a risk of significant population migration to alliance. ON THE OTHER HAND, some HE supporters claim that the number of HE players would be low (because of possible rep requirements/achievements). If this is the case, then why should another BE model be given to the Alliance for a so called minority? Furthermore, this goes against the claims of some HE supporters that "there is a big demand for HE". So in summary, we could possibly have large numbers of players moving to alliance to play HE (ie. possible faction imbalance), or we could possibly not have many HE players (ie. AR race slot wasted for the Alliance).
4. VE exist. Imo, adding HE could intrude on VE development. A case can be made to argue that it can go along with VE development.. but on the same hand a case could be made that it might possibly steal the spotlight.
5. HE might be wanted by some, but what about the rest of the Alliance playerbase? How would the rest of the Alliance playerbase feel if ANOTHER BE/VE model is added to the Alliance? When that slot could be given to other races (such as Furbolg, NE Worgen, Mechagnomes, etc...) that could provide more variety of choices, as opposed to a blue eyed BE or a pale skinned VE. Just food for thought.
6. What equivalent would be given to the Horde? Some have suggested undead elves or Sen'Layn.. but neither are even remotely as frequently requested as HE. Plus, they are effectively just re-skins of existing Horde models, whereas the HE would be re-skins of the Horde BE model (ie. Horde model). See Point (1) before you say "it's an Alliance model now hur dur". To 'balance' the trade, imo, Undead Variants would be a fair trade. Variants as in the forsaken can choose to be an undead human, elf, gnome, dwarf, orc, etc...
7. Half Elf should not be included in customization options for HE. Imo, half elf and HE should be separate AR if the both are to be considered. Putting both options in customization would be an unfair inclusion.
So, if Blizzard are considering playable HE, I would humbly request that they consider:
1. the fact that you would be imparting a part of the Horde identity to alliance.
2. there could be a possible (emphasis on possible) faction imbalance due to HE. This risk should be considered and a suitable control measure implemented (this could be done by implementing rep requirements, only offering limited class options to HE, only offering limited customization options to HE)
3. the rest of the Alliance playerbase. How would they feel getting another BE/VE model as opposed to an AR that could provide some well accepted variety.
Before any HE supporters get offended at my comments, please note that this is my opinion on why I don't think HE should be playable. I accept that HE have a good lore foundation, have been allied with the Alliance for a while and that their presence is seen from time to time in various expansions. I just feel that adding playable HE is somewhat inconsiderate to the Horde population who take pride in their allied friends "The Blood Elves".
If HE become playable, then that's fine. As stated, I'm just expressing my opinion on the matter. My opinion could be seen as wrong to some, and right to others.. and that's okay... But before you point the finger and accuse me of "this or that", please remember that your comments are your opinion also and some may or may not agree with your opinion. I would appreciate that proper thought and design be put into this idea, if HE are to be considered. It isn't as easy as saying "we are entitled to them", you need to consider that the High Elven society has been a major part of the Horde for the last 13 years or so, in the form of BE. Blood Elves, although culturally and politically different from HE, are still part of the greater High Elven society. Compare it to the polynesian people... the polynesians of Hawaii (lets call them Hawaiin's) sailed west many years ago and landed on the shores of other polynesian islands. One land they arrived at is called New Zealand... and they polynesians of New Zealand are called Maori's, who are descendants of the Hawaiin's. Although there are some cultural differences between the two groups of people, they are both still part of the POLYNESIAN group. Likewise for BE and HE.