Even though you're using some red herring arguments, I'll still concede half the point to you. If there is a "non-evil" use for something, you should be able to use it that way. An no, you shouldn't ban everything that has a possible evil use. I agree.
However, if you can do the same thing with something else that's more effective, why use the dodgy thing?
Let's use this analogy: mustard gas. So, this was conceived to do harm (though in the idea of a perceived greater good), but maybe there are some positive uses for it. Say, as a spray to deter would-be rapists. So yes, we should be open to re-purposing something bad into something good. However, is there an alternate product, something that could do the same good thing, without making it easy for people to do bad things with it? If so, then there's really no need to make mustard gas easily accessible and "legitimate."
Bitcoins: Hey, look at all the perfectly normal commerce transactions I can do with my bitcoins! Great - hypothetically, could you do those with another, established currency? Yes. Are the other currencies more efficient to use? Yes. So, why do you want to use the less efficient currency, again?
Question (serious question): What do you NEED bitcoins for? What actual need do they satisfy?
A: Things you can do with bitcoin that you can't do with other currencies:
1) Carry over $10,000 worth across international borders without declaring them.
2) Buy a metric ton of heroin.
3) Buy 30 slaves
I'm sure there are other things you can do with bitcoins that you can't do with other currencies, but I'm not sure - need some coffee. Please let me know what you can do with bitcoins that you can't do with other currencies.