A polariser also warms up the colors a little bit, so there might be a little difference. I use it all the time when I need low shutter speeds and adding a ND filter is not enough.
I agree with Bart
If you "dial in" the polarizer 90 degrees to the sun and then shoot into the sun it does not just act as a ND as Bart says. I do it all the time.....lazy buggar I am