There are several answers to your questions.
--
All the apps that task killer apps are showing are
currently running (if the task killer app is worth its salt).
-- On the other hand, the "recent apps" list you get with a long-press of the home-key lists just that:
recent apps, whether or not they are actually running.
-- There's really no problem with having lots of apps running at the same time. Rejoice, it just means you have the memory to do so!
-- Android works in such a way that foreground apps should never be forcefully closed by the system, but background apps
may be closed
if the system needs to free memory for foreground purposes. It's similar to those who complain that browsers in Windows eat up hundreds of megs of ram -- why not, if nothing else needs it, and you still have ram to spare, there's really no harm in it.
You
probably won't benefit very much from killing apps, as they're likely to restart instantly or at least eventually.
Examine the type of apps you see listed, and consider if it's reasonable. If you have a lot of live widgets (weather, social, device stats, etc) then they all will have a process to keep themselves up to date.