I'm a pretty active user on Sythe, and with that rule intact, you pretty much learn to ask for a PM before you even ask about a trade.
Sure at first, there will be many fakers etc, however in like a few week's time everyone learns to just ask for a PM. Of course you'll get some people still not asking for PM, and claiming they were hacked etc, but no-wheres perfect. We still get that over at Sythe, but it's still a far more secure system.
I support this 100%.