Bitcoin should exist for the same reasons Tor exists. Just because Tor is used by child predators and of little practical use to the average Western citizen doesn't negate its positive value.
I think at the end of the day, it comes down to the fact that with every new tool comes the opportunity to use it for good or evil. So I wouldn't blame the tool, but rather the person who uses it for good purposes or for ill.
you cant just say that and expect me to decide its worth it on balance, you need to make a case for it. maybe this isnt the place, but its a funny thing to just kind of assert. arguing that things should exist because there are potential upsides is missing the point, which is that the downsides may outweigh the upsides. its particularly bold to make that claim about tor- there must be a hell of an upside to justify the child predators...
That is reason enough.
Even if Tor didn't exist, there would be child pornography. Would there more, less, or just as much of it? That's difficult to say. It's also difficult to say how much easier it makes the lives of those fighting for freedom of speech. Thus, saying whether Tor is, overall, a "good" or "bad" technology would involve comparing two unknown quantities.