My main problem with it is the spyware usually runs even when not using the program that installed it (and often hangs around even if you uninstall the original).

The people who write the spyware itself (I'm not talking about the programs that bundle it necessarily) care nothing about the users except how they can further exploit them. Most of these programs are very poorly written, eat up your system resources, and cause crashes and instability galore. I imagine many are also extremely insecure and full of holes that could make you vulnerable to any half-assed 133t h4x0r.

To me they're as bad as a virus or Trojan and should be just as illegal. Nothing should ever be placed on a system without full user consent. These programs absolutely rely on users being unaware of how they work and what they truly do.

Software developers that use them are making a deal with the devil - one that will always cost in the long run. It's basically like a giant neon sign to all future investors saying: "We have no legitimate business plan or source of income".