The main issue that I see with going for the prohibition market is that this undermines the attempts of Bitcoin to establish itself in other markets. I'm not going as far as saying that you can't have both at the same time, but there is a certain interrelationship. A lot of people support the war on drugs and so do the legislators. If Bitcoin tries to position itself in the prohibition markets, these people will be less inclined to use it and even attempt legislative attacks. The legislative attacks might not succeed (particularly if the fiat currencies collapse, as Schlichter rightfully pointed out during the panel talk at the Prague conference), but it's something to consider.
firstly, let me answer to catfish: you should not be ashamed for predicting and/or suggesting bitcoin conquer some illegal markets. You're not promoting breaking the law, so why should you get banned here? Even high profile politicians do this:
http://falkvinge.net/2011/06/16/bitcoins-four-drivers-part-one-unlawful-trade/lonelyminer: As you know, I have high respect for your opinions. I'm not sure about this one, though. I lean a little bit to the other side, thinking that bitcoin being used for unlawful trade will not by default have a negative impact on it's success (namely it's use in other, more legit markets).
Let's assume for a second that bitcoin is used
widely in unlawfull black markets (drugs, botnets, services, human traficking, human organs, whatever sick shit you can think of). I'm assuming it already
is being used in some of these, just not widely. Now the prohibition of use in other markets would have to take the way of either
a) shaping public opinion negatively (smear campaign) or
b) legal action.
b) will fail to take down the currency itself, of course, because bitcoin is specifically designed to withstand such attacks. I think it will also fail to make a big enough dent into legitimacy of "legal bitcoin use". I just cannot imagine "bitcoin being made completely illegal everywhere". This might be some sort of a gray area and this attack then becomes more like a), an attack on trust.
as for a), let me argue that we have seen this on a smaller scale already (remembers the congress member xzy talking about silkroad/bitcoin/(tor)). This was during the summer bubble time. Now one could argue that the slow steady decline of bitcoins price (an indicator for its success) is at least partly due to that negative press. I think, however, that the mybitcoin fiasko and gox hack (however unfair towards bitcoin itself) had a much greater share in the doomsday reporting by media. Did silkroad have negative impact on actual "bitcoin use in other markets"? I think you'd have a hard time arguing this.
So in summary I fail to fully see that use in unlawful markets would prevent bitcoins adoption in other markets. I'm not dismissing the reasoning, just leaning to the other side thinking: "noone cares USD cash is used for drug trade, why would anybody care in the bitcoin case, as long as it has relatively stable value". I for one would put much trust into bitcoins value if the mafia or any such "organization" used it for inter-familiy bookkeeping and/or store of wealth and/or trade and/or settlement of debt.
After all:
Geld stinkt nicht. ("money doesn't smell")
EDIT: I do not promote illegal activity.