I would imagine some apps umm, crash sometimes? or rather stop working and keep hanging in the "working" stage... thus taking up memory. In cases like that I'd use a task killer app, surely.
I also understand the notion of having lots (LOTS) of memory and not needing to use taskkiller or similar apps, but I personally like to have my phone, and pc for that matter, absolutely clean of any unnecessary processes. Yeah with gigs of memory you shouldn't notice any decrease in performance, but that does not mean some "garbage" is not there
also, even if you close an app with a TK, the OS will load up apps on its own, supposedly it learns what you use and tries to anticipate your needs so that when you inevitably click on the app, it opens faster.
i'd try going without it, and if you have issues then install one.