I go direct mostly because it's easiest. As much as I'd love to Mic it, all my mics are cheap and crappy. Just not worth the trouble for bass - I have a feeling it would sound thin. Maybe for guitar because I want to hear the garbled distortion coming out the amp.
In the future though, I'd probably do Mic and Direct at the same time (I have two inputs in the ol' Tascam US-122). I only record for me anyways, and the random person that stumbled on my Soundcloud music.
You can get some pretty nice recordings with just an SM57! I like the combination of an SM57 and an RE20 - a few hundred total, used.
It's only really easier to set up direct if you can't leave your cab and mics set up from a previous session. Once you've mucked around adjusting the mics and have got the sound you're after, if you can just leave it like that, all you have to do is arm three tracks instead of one.