It seems unwise to get into an arms race with one’s players unless a certain kind of forensic play appeals to everyone. If so, power to you. But this is not to everyone’s taste. I like games to focus on set-piece battles, cinematics, and character development. There is somewhat less touching everything with a pole. We kick down doors because, dammit, Aragorn never listened to a door and puttered around it first.
Then you might as well just deal a little damage to everyone every few rounds then and not bother with letting the players come up with solutions that actually work. If you are just going to escalate things to make sure the players step on the trap, then just be up front with it and deal the damage already.
This, for the record, is one of the many reasons why my group usually just hates traps and such. They are a novelty every now and again where the fit the flavor. But every player and every group has a different style. I don’t mean to criticize you so much as present an alternative viewpoint.
No matter what your style, as a DM, Dispel is usually a bad idea. If I induce an enemy spellcaster to burn a standard action to dispel my polymorph, then victory is mine. That’s like having a 1-round mage lockdown with no save or spell resistance. Please, please try to dispel my mid-level buff instead of Avasculating me. My caster level will usually be rather higher than most enemies’. Even if this were not the case, there would at best be even odds. Dispel is usually the last refuge for people who have nothing better to do. For a spellcaster, that should be rare. Ignore the polymorph and just Enervate or something.
This is kind of true, but only for the most egregious strategies. If your player has a strategy integral to his concept, why punish him for doing what he wants to do? A character built around polymorphing is canonical. Why should I be a dick and deny the player actually realizing this character? Sometimes I might make it a little harder, but I am not convinced there is a lot of value to countering what the players are actually trying to do. As long as the characters are balanced to each other, what difference? Just triple the monster’s HP, throw in a few extra Horrid Wiltings, and call it a day.