You know, the real interesting stuff might be to have Resistances that don't just reduce damage.
Examples:
"Resist Fire: If the creature takes 10 or more points of fire damage, it can make a basic attack or shift 3 squares"
"Resist Cold: If the creature takes 5 or more points of cold damage, its Icy Blast power recharges."
I guess I would try to incorporate these weaknesses into the monster's entries. I would still try to come up with creative vulerablities over a static damage bonus.
You know, the real interesting stuff might be to have Resistances that don't just reduce damage.
But to the real point: Why would more vulnerabilities be "bad for the game"? Let's posit that a few more monsters gain vulnerabilities, and only if thematicly appropriate. Would that be bad?
For example: Some newbies that I've DMed threw water at Hell Hounds, as the hounds were partially made of fire. They thought (being new to gaming, and having never read thru a Monstrous Manual) that water would hurt the 'Hounds. Wouldn't it be nifty if water did?
The newbie player sure thought so. The grizzled veterans (this was 3.0e) kept saying: "That doesn't work".I think it'd be awesome if you could douse the hellhound's aura for a turn