tl;dr:
ML is kept around for older mods (FC, I'm looking at ya) that don't want, for whatever reason or another, to use forge. It uses some 'hacks' to go around the integrated server. So no ML mod is SMP. They only work in SP/LAN.
MLMP was/is still on 1.3.2 last I checked. So no one should be using it.
MCPatcher plants shit all over the place. It doesn't replace the classes like a good mod installer, it uses voodoo magic to insert its patches, hence the name. Also, the Utils are also installed even if you select NOTHING, and it does tend to screw shit up, especially with FML HD texture support.
CraftBukkit: Read this and you'll understand: http://forums.bukkit.org/threads/forge.109356/