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.
Besides the point that it wouldn't be possible to use mustard gas for this purpose: Regular pepper spray can be used to deter would-be attackers. (Let's downgrade this to "attackers," make it a little less open to a sympathy/emotional argument. Besides, why limit it to rapists, why not all potential attackers when you're hanging out in a less-than-safe place at night?) It can also be used as a weapon by "bad guys." Can you name a single thing that can be used to deter an attacker that can't also be used by an attacker? Maybe, like, a dog? But a would-be assailant could always train an attack dog as well. Bitcoins - I don't use them. But I feel your entire argument discourages innovation and discovery. Why do I "need" to have something in order to justify its existence? I can't justify my own existence. I am just lucky enough to be on this earth. Do I need to prove in some way with my life that I "deserve" to exist? (Answer: No.) Bitcoins are an innovation, an experiment. A "let's see what happens if we poke this thing" sort of situation. You are basically saying that if something exists that fulfills a purpose, why create a second thing that can also do that thing? My answer is: why not? You don't know how you can improve upon an existing product unless you try new things. That includes creating failures. And, by the way - initially, you could get bitcoin for no cost or (really) for very little work, which was done by your computer. Can't do that with other currencies. We should be able to experiment and we shouldn't have to justify our experimentation with "efficiency" and "purpose" and "reasons to justify its existence." Consider Bitcoins a piece of economic art, if you must.
I'm not at all saying that if you have one thing that works - don't innovate. I am saying "don't adopt." I'm already a bit gun-shy about using any parallel example, as they (the examples) will be picked apart, and not the actual topic - but here goes... If you have DVDs (and DVD-R, etc) and someone invents VHS, then it shouldn't be the next big thing. Especially if VHS was invented for creation and distribution of child porn. Yes, you can record Night Court on it, too, but the quality is just not as good - so why adopt? I know it sounds like I'm trying to sit on some moral high horse here, but I guess what I'm saying is I just don't see the point. I think the only people who should be using bitcoin are the people who need to use it. Everyone else should carry on with using something else or driving further innovation.
Your analogy is flawed. Bit coin was created as an economic experiment, not solely so people could buy drugs off the Silk Road or anywhere else. The crux of the biscuit here appears to be that you think bit coin was created specifically for nefarious purposes when it was created as an economic experiment. It's much more akin to VHS created and then someone decided to use for CP than your example where it was created with that use in mind.
OK, perhaps I did have some bad intel on the why it was created. I thought I had read that somewhere. Maybe that was why Silk Road was created, rather than Bitcoin? Or maybe neither, but that's why one or the other got "picked up." I can't find the article now (I read it over a year ago.) Anyway, I'm still baffled that Bitcoin is getting any traction in the mainstream. Digital fool's gold.