You can pixel just as easily with Photoshop if you're comfortable with the interface. The difference is that in Photoshop, when you're done pixelling, you can drop in your favorite photo and do some
serious manipulation... let's see you do that in paint
For all practical purposes, MS Paint is better for the average pixeller. If you're not doing photo manipulation than Photoshop will be nothing but a filter-trip for you. Besides, Photoshop is expensive/long to download.
I did a little detail on a pixel job Tom did a few days ago, simply using some brushes. Came out pretty great. The original looked nice, but with the addition of depth and gleam made it just pops out at you... it's a lot easier to add that depth in Photoshop than it is with Paint.