IMO, a lot of it depends on the type of sound you are after. Some guitarists like to record a clean electric direct in to the mixer so they get just the sound of the guitar and not the room. You'd probably still use a pre-amp, because the signal from the guitar is too weak to record directly. For a distorted guitar, most people prefer to mic the amp, because the speaker is an integral part of delivering the distorted sound. If you bypass that by recording direct, it's harder to get the bite and crunch you want. Of course, this also requires (1) a decent mic, and (2) an environment where you can turn the amp up loud enough to overdrive the speaker; that can be hard to come by.
Hope this helps,
David