Good Idea and Bad Idea.
Good:
Less raiding to one character. If you wanna raid the entire week, this forces you to gear up and prepare other characters... mmm alts becoming nearly mains... I dig that.
10 mans becoming preparation and practice for the 25 mans to gear the whole raid/guild. I think were going with the attempts strategy of the hardcore guilds (lvling alts similar to their mains to get practices and not wasting attempts or in this case, bosses). I like it.
25 mans are NOT gonna die. they will become a more enjoyable experience overall.
Bad:
Balancing nightmare which will take weeks / months to get right. ( Hope beta gets it right.)
Those with only one player, are screwed. They might as well prepare to level another to keep their raiding experience intact.