[ben@tirerack]

Lots of good info here. Like most things in life, whether or not spacers are right for you is not a simple yes/no.

A couple things to add:

Sometimes spacers are used to bring a wheel that does not fit right into the proper position. In a case like that, none of the concerns about scrub radius, road debris, rubbing etc would apply.

Any of the issues associated with the wheel sticking out further would also be applicable to wheels that are wider/have more aggressive offsets.

The unique issues of the spacers are those that have to do with the use of the plate in between the wheel and hub: some added hassle on install, more stress on lug bolts, need to get longer lug bolts, etc. Some spacers are thin enough that the wheel can still center on the hub. Some thicker spacers have a pilot or 'nose' that centers the wheel. A thick spacer with no pilot should be avoided.

Think of spacers like you'd think of forced induction. They have their place, they are useful when done right, but you can break things if you are not smart about it, and they do add some complexity.