Font, speaking only for myself, some of us are just scatter-brained and either can't decide on using just the one vise for a favored fly type or just have this want to have a few nice tools around and switch it up. Either that or as I mentioned before, some flies are more enjoyable to tie on a particular vise - e.g. tiny flies on the midge jaws - or the fun of playing with diff. toys.
It's part of the enjoyment of the game, much the same as bamboo rods. Variation. I fit all the above categories - 'specially the bamboo thingy - and most notably the scatter-brained one!
What can I say...