The raid composition for a 25-man raid is much more similar to the proportion of people who play the different class roles. You also have a better chance of getting all the raid buffs and utility covered with 25 random people instead of just 10.

I think this is what Blizzard's reasoning was.

You could never run a 10 man version and a 25 man version at the same time. 25 man queues would go to the mooooon. 10 man queues wouldn't be any better...

You would have a similar problem with queues for dungeons.. not enough tanks/healers

10 Man: 25 Man:
2 Tanks 2 Tanks
3 Healers 5 (or 6?) Healers
5 DPS 17/18 DPS

This keeps the queue times lower. Sure you're more likely to run into undesirable people of all shapes and sizes... but you're going to be waiting considerably less.

Oh boo. Guess you cant space things out.

I understand the proportional argument that 25

You are more likely to have at least one semi-competent leader among 25 people than among 10.