I would add some light weight-long reps to the equation. Weightlifting can work opposing sets of muscles to the ones that you train aikido with. For instance, your quadriceps can get very built up from taking nice, deep stances. If you don't also work the hamstrings, the imbalanced forces can pull your knee to pieces. Tony was right on with the statement that overly bulked up muscles won't do your aikido much good.
Tight muscles can lead to injury.
I've been weight training at least as long as I've been training in the martial arts and have yet to experience any negative effects from it on my karate performance. However, when I make a conscious effort to relax when I am reminded of my mistake, I think I am able to relax much easier than I would be if I had never weight trained or, at least, not very much. Actually, though, this is a valid concern. However, if one takes time to stretch completely after the weight lifting session the negative effects can be limited, if not counteracted. In terms of musculature, weightlifting is pretty universally acknowledged to be an entirely good thing, provided it is done right.