Wheel spacers?

I'm confused. Why not just buy a set of small bolt pattern rallyes, new or used? For the price of good, billet adapters, not to mention rallyes with a different offset, you could just buy a set of rallye wheels with either the large OR small bolt pattern...

My preference is for 15" ralley wheels. Only aftermarket special build wheels are 15" SBP and they are expensive. The question is based on the premise that with the adaptors I could leave my car SBP and use 15" Ralley wheels.
Rat