Oh my god, why does even MERCHANT REPAIR lower maximum durability? No! Bad! I guess my Rogue is going to have to switch to bows because daggers break so fast. Maybe I'll get lucky and find some more drops.
Gyson, not being able to repair stuff after a certain point may be realistic, but "realism" is a really silly argument to make.
The problem with not being able to repair even after a certain point is that any time you miss repairing an item before crossing the invisible, never-disclosed line where you can suddenly repair on one side, and on the other you can't, you're forced to find an NPC and pony up cash anyway, which makes taking Repair yourself basically pointless. You are punished, where instead taking points in Repair should be a benefit.
The downside of putting points into Repair is that you are not putting points into other abilities. Period. The same downside as EVERY OTHER SINGLE ABILITY. That's all the downside you need.
Repair is a SECONDARY SKILL, not a primary one, and players who take it should not be forced to pump it as if it was a primary skill.