> Two things that weight training doesn't address is inward shoulder rotation/slumping and "turtle neck," something all people working at a keyboard are prone to.
On the contrary, weight training will address exactly that, provided you balance pulling with pressing. Most "gym rats", as you say, seem to do nothing but bench press and curls. The bench press will, if not balanced by pulling, exaggerate the internal shoulder rotation.
Deadlifts, Rows, Pullups (or lat pulldowns if you must) -- all strengthen the back, and offset the shoulder rotation issues.
> Apparently squats and deadlifts will give you pretty extreme imbalances in the hip muscles (that's what the posture specialist and a deep-tissue masseur told me after working on me). You can fix that by stretching. I don't know if it matters, because it doesn't seem to affect me functionally or aesthetically.
This is also not true. Many gym trainees who actually attempt to squat will do it with a very narrow stance, and a very shallow depth. This makes it a quad dominant exercise, which could be a problem if they didn't also deadlift. However, a full depth back squat is a very balanced exercise, and will do wonders to increase hip mobility, not to mention strength. Deadlifts are the single best exercise for strengthening the posterior chain, which anyone who picks something heavy up off the ground needs.
Many people avoid squatting and deadlifting because they think they will hurt themselves, so they end up doing much worse things: leg press, smith machine squats, hack squats. Instead, learn the lifts properly ("Starting Strength" by Mark Rippetoe is a fabulous book for this), and you will realize how valuable they are.