Let me give a real world example.
If i were to sell 50 - 100 btc worth of cocain, and i received 100,000 cockcoin or dickcoin - i would lose alot of fucking profit dumping into those buy walls, Because lets face it, at the end of the day I want bitcoin...
Simply, these coins cannot scale to transactional use because the frequency of transactions in a niche black market [cloak, drk] is far less than the frequency of transactions that take place on a gray market [bitcoin]
You may be correct right now in regards to liquidity, but if crypto currencies are more widely adopted in the future that very well could change. Even now.. take BitsharesX for example.. they had a volume of over 1 million USD today. Sure, it's much more volume than they are likely to have on any given day in the near future, but that is a lot of liquidity.
Anonymous features are not enough to get a crypto currency adopted and to get people to invest in it, you need some other improvements or features. This is what Bitshares did right with bitassets and Dpos, along with Titan (the anonymous feature).
http://wiki.bitshares.org/index.php/TITANBy adding more improvements/features, they can successfully bring in several types of users/investors:
People that like PoS
People that like financial privacy
People that see the value in bitassets
And perhaps people that see the value in 15 second confirmations
In summary... an anonymous feature does not have to be the only selling point of a crypto currency, which would certainly limit the scope of users/investors it can attract, and overtime as crypto currencies are adopted more widely liquidity will be improved across the board for all crypto currencies that are not "dead."