It's annoying, but it really cuts down on the spam, both the jackwagons who list and repeatedly bump their insanely overpriced items, and the scammers & spammers who contact sellers. Sucks as a buyer, but it's only $4, so if you see some things you like, not a big deal. Also still free to peruse, and most vendors have their business listed in the ad, which you can look up and call or email directly.
I would like to see them do something to the effect of 3 or 5 free contacts per month for buyers, though. But then, the spammers & scammers would probably just use bots to jockey around fake IPs and re-register, so that may not work well, either.
For as much as people like to gripe about Armslist, it's a good thing to have, if for no other reason because it helps maintain our culture's presence. There are certainly deals to be had out there, and plenty of reasonably priced stuff. I've bought & sold quite a bit on Armslist. Recently picked up a really nicely sporterized Winchester 1917 Enfield for $240.