I'll echo the above comments, and add: I have more than a few TC's, which I don't use here, but who knows? Maybe someday I will have a NEED to put my Spiratone 400/5.6 on two each 2x and 3x TC's for a 4000mm f/56 lens experience. Sure, that could happen.
Most of those TC's came in batch-lots amidst more desirable glass. I've sold a few for around US$6 each, both M42 and simple PK. I recently got a PK-M Panagor 1x-10x variable macro TC (CHEAP!) to play with. It hopefully won't perform any worse than +dioptre closeup meniscus glass.
A TC matched to a specific lens is one thing -- all telephoto lenses have built-in TCs, and all wide-angles have built-in reverse TC's. A TC unmatched to specific lenses, designed to work somewhat adequately with any lens, is an entirely other thing.
Bottom line: Some TC's (especially high-end), matched with specific lenses, do quite well, or so I hear. I would use an average (cheap) TC on a long tele in circumstances where the image
content is more important than its
quality: surveillance, blackmail, target-marking, etc. Any captured image is better than any missed image. Many TC's are now cheap enough that they can be experimented with, tried on different lenses, without too much financial pain. Or just buy a cheap mirror for that extra reach.