Well, for example when I go into Wikipedia frenzy, I can easily open 10-20 tabs, terminology, graphs, tables, nested articles etc. It's much faster to switch between tabs than close and open them constantly. Why should I limit myself to 5? And yes, after an hour or two I usually get them off the stack one by one, collapsing back to first one, knowing much more about the history of Parliament in Canada.

"I say, let your affairs be as two or three, and not a hundred or a thousand; instead of a million count half a dozen, and keep your accounts on your thumb nail."

I bet Henry David Thoreau would say the same about browser tabs.