This was my question as well. I have seen it a lot. I understand it for people that have vim ingrained in their fingers (muscle memory).
I do not understand the reason for this at all for people who don't know vim. There are plenty of other editors that are just as feature rich, if not more so (Sublime and PSPad come to mind). Also, if you need the GUI based macvim, you lose the most important feature of vim and the actual main reason for its popularity today: the ability to edit files locally or remotely inside a shell that supports only text.
Then again, choosing inferior technology is actually quite common these days, as the widespread use of Rails easily demonstrates.
MacVim can have all of it's toolbars removed (that's what I do). The only reason I use MacVim over vim in a terminal is because I hate hunting for my editor in all of the tabs I generally have open in my terminals...
I'm on Windows and Linux, but my reasoning is similar. I also occasionally use the mouse for text selection. On Windows I especially like using GVIM over VIM because of the extremely limited font selection for the Windows terminal.