Agreed. We clear ToC 25 EZ with 2 groups each week -- 1/2 mains, 1/2 alts. We then use all mains for ToC 25 Hard.

ToC was fine as it is because it was short. If they would continue to do that on Icecrown that would be major time waste for some people and would burnout a lot of people. I mean if theres 12 encounters and 4 different lockouts (10man, 10man heroic, 25man, 25man heroic).. would you want to do 48 encounters and for how long?
"Today 10man for alts/new guys and you need to come because you are in major role in guild(MT or healer etc), then later 10man heroic. Then we do 25man tommorow for everyone and after that that we try 25man heroics. You must attend or thats less dkp for you and you are the one making it happen".
That would be nightmare for smaller progress guilds and future farmers. Specially for guilds that would still raid ToC+ToC heroics (why not, people still raid ulduar for pesky slots like weapons, trinkets and offhands/shields).

Don't make yourself feel like you need to farm everything. That was not how it was meant to be played in the first place. Raid the way you enjoy yourself most. Sadly, many 25-man guilds go for overzealous farming of the same fights over and over again on different lockouts, while they could have imho more enjoyably spent their time trying new fights or hard modes on just 25-man, while on a voluntary basis letting those who honestly *want* to farm 10-man do so outside of guild raid times.