Remittance not useful because of currency exchange spreads? Well then no currencies are useful for remittance.
As for the illegal part, are US dollars not useful for Drugs, Gambling, Tax Evasion and Escaping capital controls anymore? So then, when was the last time some one used US Dollars for any of the illegal activities above? The news didn't reach me.
But as for illegal uses, the US dollar is useful for drugs, gambling, etc. specifically because it's very useful in legal transactions as well. There's a chicken and egg problem: most people don't engage in illegal activity, but right now if you use BTC it's a red flag that something illegal is going on. If you want BTC to be really useful for illegal transactions, it needs to be useful for legal ones and gain wider adoption.
That was my reading of it, anyway.
BTC would be a great intermediary currency if there wasn't one already in the form of USD. I just don't see any advantage that BTC would offer over USD, even in the presence of high levels of BTC liquidity (assuming nothing catastrophic happens to the USD, of course).