Florin, I wish we were
Are you saying they fix and modify things that aren't broken? That is not always a good thing, you know.
In our case it's usually always a good thing, but to explain I have to put it in context. The developers I work with deal with a pretty large application - about 50 man years of development and probably around 1/2 million lines.
During the process of making a mod, one might find things like "magic numbers", spaghetti code, things that need to be modularized, etc. Even though our product is based on a state machine (something that really helps it's stability), as it grows, sometimes parts of the foundation need to be reinforced.
We are a professional shop, and it's generally the developer's responsibility to do unit testing. From there it goes to QA where they do regression testing, functional testing, etc.
As far as the added cost for this, we eat that. But better code usually costs less in the long run.
Sorry! I'm at work. :-X