Expecting Change
For me one of the reasons I liked night elves was because I loved the narrative of pre-sundering night elf dom, they kick off in what seems the first major change in 10k years and for me it felt like this was going to be a story of a return to glory days of old... they had put aside sorcery for awhile and now the reason was ended - one of the first things I thought would happen immediately after WC3 was that the night elves would start practicing the Arcane and start rebuilding their ruins, but instead they seemed to just hang in limbo... I had almost given up hope, but now in Legion, with the revelation of the nightborne, they've brought alive ancient night elven culture back. This is what I wanted to see, and this was the part I liked most about them. So I'm happy with this new direction of the group. And if they became playable as a separate group or a night elf sub-race, I shall probably like them most of all the groups - provided they don't make them evil or cowards, which judging by Thalyssra, they're turning out to be quite the cool group.
Some people don't lilke the change their races go, sometimes it's so much they fall out of love with it. When the night elve stopped being cool like they were in warcraft 3, many people lost interest in them. WoW at least has tons of races you can switch to, and you're bound to find one you like. Even in races you may not like, they may have a sub group or alternative group you may like instead. Draenei group is a good example, as are the night elves and nightborne. With the Draenei group, you have the really good righteous ones - if that's your thing you'd love them, but then you have the more nature/elements attuned ones in the broken, that could be more your thing, then you have the Man'ari Eredar the powerful but evil sorceror types And that might be your thing. instead of the other two.
This is one of the reasons I would really love sub-races - because they give you an alternative to the main race. Maybe you never liked the orc quite, would you prefer the slimmer, upright backed, human face structured half-orc instead (like Garona in the film - with nuch smaller tusks than a full orc) different temperament etc. You might do, and person who'd never considered playing orc, might actually really love to now.
This is the potential sub-races have to unlock for every group, and this is why it's always a good thing that blizzard continues to update and expand on the lore of a race, adding new elements, new groups etc, and with sub-races if they do it, would be an attempt to make a playable version.