An affirmative law saying "yes, bitcoin is legal" would be wonderful, would it not?
Not necessarily, no.
First of all, bitcoin is already legal. And besides that,
this text shows an example of an activity that is better off in the current prohibition scenario than what it'd be in a legalization scenario. The example in question is prostitution houses in Brazil. They're currently forbidden, but if you want to open one, all you have to do is pay the appropriate bribes to the cops. According to the text author, who happens to have some friends in the business, these bribes are already lower than what taxes alone would be had the activity been legalized. Not to mention stupid licenses, labor laws, regulations etc. And if you think that being prohibited forces these business to be "shady" and dangerous, well, take a look at
this or
this or
this. One of them even has bowling lanes inside!
"Legalization" is not always a good thing, because often it comes together with taxation, cumbersome or prohibitive regulations, licenses etc etc. I wouldn't like to see bitcoin transfers crippled with the same kind of AML bullshit that affects fiat transfers, for example.
A good read:
Air Guitars and Bitcoin Regulation.