I apologize for the tone, it's more just irritability. We've been working 18 hour days for the last week (Army).
That said, different enemies have different AI routines. The Hybrids WILL move around rocks for cover and flank you. It's hard to see because, well,they generally kill you when they flank you.
The Grims are half bred chimera. They have a "swam" AI set, basically they rush in packs and attack, that is how they act. Insomniac took the idea of "crawlers" that you generally see in FPS titles (small creatures that you aim at the ground and shoot as they rush you in numbers) and made them full sized creatures. The end result is Grims.
Then there's Steel Heads, these are the guy's with the Augers who shoot through walls. They will generally "stop, pop, and rush". When they appear as regular enemies in packs of 10, the AI is great, as they shoot, take cover, and rush. They are constantly closing in on you. When you see the Primarch though (the boss Steel Head) the AI seems "stupid". The reason for this is because by the time you actually fight through the enemies, there are 8 of you shooting at him. How do you really expect the AI to deal with 8 different people attacking it? It has to decide who to attack, what to do, where to turn, walk, etc. It's not really as easy as you're trying to make it out to be.
As for the Medics, you really should switch up your play style. I generally have about 50 kills as a medic, ranked at the top. It's probably one of the most hectic classes in the game. You have a secondary weapon (the Carbine) that you can use to kill enemies, along side your primary weapon. Really, playing with just one weapon is rather boring, and I guess I can see why you're not having fun if you're just going through the motions. That's like saying Gran Turismo is boring, but you only use one car for all of your races. I can imagine that's rather boring.
Each class has strengths and weakness's, but the fun isn't in hitting the switch, it's in playing your role. Being a medic and reviving your team mates, keeping them healed, while killing chimera is a rush. Being a Spec ops and teaming up with a soldier to clear out a room of chimera, feeding him ammo and racking up kills is great. The FUN in Co-Op is in the team effort REQUIRED to win, and the leveling and unlocking of new weapons. As you level in each class, you gain access to new secondary weapons, new abilities, and new armor / items to increase your stats.
Really, it just seems like you've given it a few minutes to play, and came to this conclusion. There are more enemy types, more maps, more objectives, more weapons, a lot of things you really haven't seem to have known about, and you're passing judgement.
Kind of unfair, if you ask me.