Yes, the more sides you use the less the bezier error is noticible. Do it with less, a square and notice the results.

Interestingly enough, Bezier curves can approximate a circle but not perfectly fit a circle.
FROM http://stackoverflow.com/questions/1...%A9zier-curves

The only real world application I see this mattering in, but not a great deal is if you have contraint set to 45 degrees, holding down Ctrl, draw a line horizontal, at 45 degree restraint and then vertical. Change the 45 degree line to a curve with smooth join. This doesn't give a true circular corner. The rounded corner feature on a rectangle gives a far more accurate circular corners.