I belive that is when the valk gets a rare war pike so it can deal some nice dmg as well as tank. Mostly I know it cause almost every post or guide Ive read says you vant valk lvl 17. Questin is still if that goes for PvP as well.
At level 17, a valkyrie gets a rare war pike, which can do great damage, especially if you get a lucky spawn and get high ED on the pike along with the valkyries very nice boost to damage at high levels. That's why most of the guides suggest getting valk to level 17 after +skills. She gets a couple of equipment upgrades after 17, but nothing that helps out too much. A higher level valk will get more life, damage, and attack rating, but a level 17 valk will survive quite well in most every situation.
As for using a valk in PvP, I wouldn't really recommend it. Her slow attack speed prevent her from doing serious damage to anything, and she'll just provide a target for other characters to leech from. Any caster character won't be standing still long enough for the valkyrie to have any chance at hitting them, too. The best she could provide is a distraction, but overall, I don't think valk is very worthwhile in PvP.