In fact, I bet the PCs themselves find ways to slap vulnerable onto critters, at least once they reach Paragon tier.
I REALLY dislike this about 4E.
PC: "Ha ha, you are now vulnerable to cold and guess what? I have a cold bow."
This seems so gamey and anti-fantasy. Anytime they add a feature like this to the game, they should add a fairly easy to acquire defense too.
BBEG: "Think again bow boy. I drank my anti-vulnerabilities potion today."
PC: "Yikes!"