My guess of a likely reason to completely evac the station is one astronaut gets hurt, two healthy astronauts land with the sick one, then in a stroke of incredibly bad luck another astronaut gets hurt. For hurt, substitute in appendicitis, broken arm, heart attack, severe burn, poisoning from some kind of leak (ammonia?). If you're looking for a hard sci fi plot line, maybe major system malfunction hurts one astronaut, then after the medivac, while making duct tape repairs, another gets hurt, then the space pirates arrive and take charge of a derelict abandoned vessel, or whatever.
I would guess that docking with a completely dead powered down archeological relic of a station would be much more exciting than docking with a powered up and running station that merely happens to not have any humans onboard.
Station without power would start to tumble. Makes docking much harder. This actually happened with Salyut 7: the station died on orbit, no electrical or anything. The pilot of the rescue Soyuz matched its spin to dock manually. Effected emergency repairs while wearing fur-lined winter clothing.
It must not have been tumbling too badly. Seems like once you have significant motion in more than one axis it'd be pretty hard to match up. At that point, you probably have to grab and despin, or something. Or dock really really fast and hope your docking mechanism holds up to the stress.
Trying to fix/salvage it? I'm sure the russians reactivated a "dead" station at least once, sending a better prepared crew to repair quite a while after it was abandoned.
