I think that checking in commented out code should be valid as, just because the new change passed tests it may be more helpful to see what was there before and see if the new change is really an improvement.
If I have to go back several versions to see an earlier change that now leads to a performance hit then that would be very annoying.
Sometimes the commented out code is a good history, but, put dates as to when the code was commented out. Later, someone that is working near there can just delete the commented out code as it has been proven not to be needed.
It would also be good to know who commented out that code so that if some rationale is needed then they can be asked.
I prefer to write new functionality, ensure the unit tests pass, check it in, then allow others to use it and see how it works.
