An ACMG certified guide friend of mine used a rental rope in Thailand. He said that in retrospect, using the rental rope was probably not the best idea and that he'd plan to bring a rope and leave it there next time, but the rope he used didn't seem that bad.
It depends on your personal attitude towards safety and gear conditions. Most of the ropes are actually just fine, but if you are the type of person that stops using your rope when you get some fuzzies on the sheath then the condition of some of the ropes may scare you a bit.
Again, I cannot vouch for Chiang Mai standards, but the beach climbing lessons were very... um.. interesting to watch. You may even be better off asking other visiting climbers if they mind your climbing with them and getting them to show you a bit of multi-pitch trickery. Multi-pitch sport climbing is pretty easy though and if you're a smart person (which you seem to be) then I doubt you'll have trouble learning either way.
What really worries me is the combination of people who don't know when to question bad information and stoned teachers who assume that their clients will figure it out once they get up there.
Hopefully I'm not scaring you off anything, but I don't know what your expectations are when it comes to local climbing outfits and their competency. This is going to sound bad, but I wouldn't trust any of the thai instructors (that I met at least) to give decent instruction. A foreigner who has moved to the area for climbing will probably have a much more standard approach to safety and thorough instruction.
In other words: be careful, but don't worry about it too much.
Sorry for the lack of solid advice
