I believe this can happen simply from age and mileage, but most likely more from cars that have been sitting for long periods. The seal gets dried out and leaks the freon out. The seal itself it about $20 and you need a $12 bullet-shaped tool that fits over the end of the compressor shaft to allow the seal to slide on without damage.
You may want to do change this when you have the compressor out and the clutch removed. There isnt much else to remove (I believe possibly a C-clip) once the clutch is out. I can look up the part numbers if you cant find them.