I would like to believe that if they went to 15 man only, it would give them more time to focus on the content and mechanics and coming up with new ideas instead of having to spend so much time on the balance of 10 and 25 man. It's nice that you can have a choice between 10 and 25 right now, but if you aren't in a 25 man guild you're basically never seeing 25 man content. 15 would be an ideal size. It might not feel like a "raid" but then again 10 mans don't currently feel like that now anyway.
If Blizzard wants to be able to put more creative effort in to the encounters at the cost of 2 different raid sizes I'm all for it.