Did you check if you can add attribute crystals after it's PvP'ed? Since the max is now 300 attribute, and I'm not entirely sure you can add the normal stones to PvP'ed weapons.
I got also PvP on my Gram (finally lol) and tried it in arena, in open field pvp, kratei etc. In all pvps it popped in 1 time so the chance is like less than 1% I'd say :( not a nice thing if I compare with other pvp bonuses from other classes that land quite often..