I'm a programmer and I think they are worth more too. It's a disaster when you have a bad business person and lose investors because there is no one to meet them, or it takes several months longer than expected to hire a desperately needed graphics designer, etc.. I've heard managers of mine mention getting 1400 emails a day, and I bet a lot of it is bitching they have to put up with about budgets and timelines, and business considerations - not fun coding stuff like I work with all day. You couldn't pay me to take a business job, but I acknowledge it is more essential to the actual business.
If importance were gauged by doing dirty work without which things would fail, then the janitor is also more valuable. But janitors are cheap because they are easy to get.
If good* PMs are paid more, it is because they are harder to get. That doesn't mean they are more important, just that the company believes them to be worth it and does not believe it can get a better deal without paying unreasonably for it.
* good means whatever the person hiring thinks it means. If the hiring manager believes in auras or psychoanalysis or nepotism, then those things will determine the hire.
