Thoughts on Audio Autotransformers?

[JayCobie]

What do you guys think of audio autotransformers as a utility tool for mismatched amps/speakers?

I bought this Carlsbro 100v line module (probably from the 70's or 80's), rated for 100w, off eBay and was pleasantly surprised that it contained a partridge-style audio autotransformer from Drake.
It works beautifully in the 2 mismatching tests I tried (4ohm output to 8ohm speaker and vice versa with different amps and speakers) so I started wondering why I don't hear of things like this being used more often.

I realize that uneducated users would be likely to fry under-powered speakers from power doubling/quadrupling but this still seems like a great (if heavy) piece of kit to have lying around.

Any thoughts?