and for 90% of apps that's great - but the one thing no one seems to take into account is shoddy programming. there are more than enough apps out there are that simply don't exit when they should, and DO stay in the background longer than necessary. also, personally - i like to know that i can end a process when i want it to end. as simple as that. i don't care if readerHD is scheduled to or wants to update it's feeds right now - i want it closed and i want the ability to do that whenever i decide it's necessary.
I am not advocating willy nilly killing all processes, and you're right that 90% or more of users will have no need of one - but to make it sound as though there is never a use for one and the android mms is a perfect solution i think is a little too far.
sorry for the rant ... i just think there's a little more to it than "shouldn't use a task killer at all"