True, the range of motion between Prime and Grimmy is similar but Prime's hands swivel, the fingers are individually articulated, he's got little moving pistons in his joints, he has pop open leg vents, shocks on his wheels, a gimmick that doesn't hurt his articulation, his skirt is made of moving panels rather than just rotating (even the seekers had moving panels) a good deal of Prime's joints are ratchets while Grimmy has a bunch of friction ones and, what's most important to me, a transformation that is an interesting evolution of the G1 transformation, not just an upscaled one. Now, I'll admit that G1 Grimlock's transformation really didn't need fixing in the same way that the rest of the MP line did in order to get to a anime accurate bot mode... but they could have easily increased his range of motion in either mode to compensate. If they had dumped the glowy hand gimmick they could have given him pull out arms and given him a double jointed elbow, for instance.
I won't argue with you on the seekers, cuz for me, they also marked a decline in the MP brand. The hip kibble and skirt severely limits their leg movement, a transformation that pretty much causes a key joint to fail... but I give it points for having an intricate and novel transformation and for having a number of gimmicks packed into a smaller toy.
Anyways, I'm not saying he's a bad toy. I think he's WELL worth the 60 bucks I paid for him. I just don't think he measures up well to MP Prime. And from what I've seen of MP Roddy, he looks more like what I came to expect from the line.