This did get answered, but, I'd like to take another stab at it.
Despite the very good points in this thread, I am still not convinced...
I guess I just don't like seeing wizards running around casting spells in heavy armor.
Those who have no problem with that, I suppose also would not mind seeing monks throwing flying kicks in heavy armor
The reason monks don't wear heavy armor is not a contrived one though. It's based on pretty solid realism. You don't jump six feet off the ground in plate mail. Period.
, rogues sneaking and disabling traps/locks in heavy armor,
Again, grounded in solid realism. Heavy armor is noisy. We don't let people sneak around while banging coconuts together either. But, I don't see why you couldn't disable a trap in heavy armor. Lock? Well, you'd probably have to take your gloves off first, but, that's not a big deal.
druids walking their pets in the bush in heavy armor,
Now this one is actually closer to the MU restrictions. Druids were wonky in that they could only use "natural" armor and weapons, but, they could still use scimitars, daggers and sickles. They kept the "no metal armor" thing in later editions, but, the question in my mind is, is it necessary? What's wrong with a druid in plate mail? We have no problems with a cleric in plate, and a druid is just a funky kind of cleric, so, where's the problem?
Now, from a practical point of view, it does make a bit more sense since most of a druid's skills would be seriously penalized by wearing heavy armor, at least in 3e. But, is it really necessary to strictly limit the druid this way?
rangers tracking foes in the wild in heavy armor, etc...?
I'd point out that this was a 2e and later addition to the ranger. The original ranger had no such restrictions. You could wear full plate as a ranger with no problems. So, I have no real issues with a ranger in full armor. Again, skill penalties would likely apply, but, if the ranger is going into a battle, and he knows that he isn't going to need that mobility so much, then why not?
I mean, is this really a bad image for a high level ranger: