The way things are right now, it is not a matter of why it would have been good choice.
It is more about how many do have the choice still to do 25s and how likely is that to change to the better in the forseeable future.
I am not in favor of 10/25 due to the huge problems this “odd” selection of raid sizes brought over the past 3 expansions.
1) Upsizing managerial problems mainly in TBC
2) Downsizing managerial problems mainly in Wrath for the 25s to also do 10 man progress (and drama from those that were left out).
3) All shorts of problems with the “equalized” model of “choice” in Cataclysm.
My preference lies clearly into having separate content with 2 sizes available, or shared content but with separate achievements, and with a size combination that ensures “direct scaling”, such as 10/20, 12/24, 15/30.
My favorite, and the one I voted for is 12/24. It is the closest to 10/25 and has a huge array of improvements over 10/25.
1) Core groups of 6, that allow dungeons with 1/1/4 setup, increasing the number of dpsers in a dungeon group, shortening queues and bring the % of dps in dungeons, closer to the % of dps in raids.
2) Direct scaling from small to large group and from large to small raid group.
a) You work with 2 small groups and then you re ready to swap to large raid group, not as it is now that you have those odd 5 people still missing.
b) You want to raid the small size, and you dont have to sit people out to go for 2 groups (25=2,5 10 people raids)
3) Direct scaling to loot. Thus you don't need to have issues with smaller amount of tiers per person in 25 and smaller amount of normal items per person for 10.
4) Better setup for the smaller size, more space for pure dps classes representation in small raid size, better scaling of the same fights in small and large group for easier and better balancing.
5) Shorter queues in dungeon finder, and with a 5 healers requirement, the same goes also for LFR.
6) Option for full class representation in MoP, with monk added 10s are not possible to have all classes in a raid, smt that was never the case for a “raid” group till now.
18 man would be a quite agreeable number to me as well, but not for reasons related to how many people we have available lately for raiding
They re hiding in Diablo 3!!! Who cares about exams
Overall, single size to me is a compromise, but again if you ask me if I prefer 10vs25 from any of 15, 18, 20 single size my answer would be flat out, single size!
10vs25 will continue to kill raiding for MoP.
For that reason 10vs25 should die instead and start fresh!