1991 Mercury Capri AC Compressor Replacement of Repair?

I found out, after retrofitting my AC system to R134a instead of R12, that my AC compressor is leaking from the front seal. Is it more common to replace the unit altogether, or fix the seal? From people I've spoken to, replacement seems to be the most common option. I obtained a compressor from a salvage yard, guaranteed to work; compressors for my car are not made anymore. Is it such the a compressor seal goes bad at the drop of a hat or just over time and continued use? I'll be getting the complete unit replaced soon. Just want to make sure that if I got the used compressor and it was tested to work, I shouldn't worry as much as I have been.