radeng, if I had suffered poor service, then yes I would write a letter of complaint. I am more than entitled to do so, having paid for a service. Of course there are limits, for example I couldn't write after every KLMuk flight to complain about the rude, abrupt and cold service, I can only hope that it is better next time. (And that the change to KLM/KLMCityhopper means Dutch crews.)
The point is that if I encounter poor service in a restaurant, shop, pub, etc. I'd attempt to tackle it with due politeness there and then. On board aircraft, we are more and more pushed into the belief that any complaint pitched at any level will land us in trouble. There is no industry on the planet where the workers have to be treated with such kid gloves to avoid any upset.
Remember, no SLF, no job, no wages. If you don't like that go and work in another industry, it's a simple solution.
For what it's worth, I would say that to anyone in any job, not just aviation related ones.