It actually depends as to why you need them. If you want a set of 4 just to get the wheels out further, then as stated, it's cheaper to buy new wheels.
However, if you are swapping an axle that has a wider width than the other and need to buy 2 in order to match widths, then the spacers become more of a reality. That way, no matter what wheels you have, the width will always be the same.
Depending on the spacer size, you may or may not incurr some additonal stress on the axle bearings or axle itself. It all kind of "depends".