I'm a D-Pad person so it doesn't much matter to me.
I absolutely love the skates! I find myself spinning almost all the time when I'm on my skates, and I really love doing the cosmic backflip. Though upon closer inspection, it looks like I spin on my shins... Ouch! And that can't be good for those super-expensive pants I'm wearing. For me, they make the bike redundant, and so I unregistered the bike to make room for other key items. I do miss hearing the bike music, though.
I would have like the option of switching skate movement to the D-pad. It's just too natural to use the joystick for moving :(
I don't think it would make sense to put skates on the d-pad. The entire point of the skates was to allow for unrestricted movement against the grid-like tile system that the game employs, and with a d-pad you still only get the 8 directions you start with.
I agree that they should be optional if everyone dislikes them, but I do see that they simply intended it for the skates to be able to do what the D-Pad should be able to do.
Hopefully in any subsequent games they either modify how the skates work or allow them to be toggled, since there shouldn't be any reason for people not being allowed to simply run around in an unrestricted-to-the-grid manner.