I absolutely agree that commented out code shouldn't be checked into the repository, that is what source code control is for.
In my experience when a programmer checks in commented out code, it is because he/she is not sure what the right solution is and is happier leaving the alternate solution in the source in the hope that someone else will make that decision.
I find it complicates the code and makes it difficult to read.
I have no problem with checking in half finished code (so you get the benefit of source control) that isn't called by the live system. My problem is with finding sections of commented code with no explanation the dilemma was that resulted in the code being excluded.
