I don't know, habit I guess? The ports on the keyboard don't provide as much power as the ones at the back so I generally have the ones at the back for things like hard drives etc.
At least it only uses one cable to power the hubs right? I have a DSI SMK-88 that has two cables so takes up two ports on the back of the iMac, one to power the board and one to power the single hub on the board (not dual or tri hubs, just the one...). If it wasn't for the blue switches in it I'd sell it on.