Well for casual 10 mans it is fine, and you would bring most buffs. Hardly any classes are that bad for 10 man that you would totally skip them.
Perhaps for the very top tier of raiding you would class stack ot an extent in 10 man but that is 0.000001% of the community. (aka the guilds in the top 10 of progress).
The size of a realm isn't that bad. On huge realms a problem is getting people to join you rather than the 'bigger' guilds, or just rather than simmilar guilds.
On small realms there is a tiny player base, which just makes 10 man unconditionally easier.
@vapo I was not talking about the BEST comp, just making an easly viable one. The shadow priest would be os healer should somebody for example not be able to make it. you can switch the spriest with a dk dps or something if that would make the loot issue better.
Who's to say that 25 mans wont run with just 1 hpaladin or 1 caster mail shaman, they could get DOUBLE disencahnt on one boss.