One never knows what are the key points he (or somebody else) will be looking for in the future. So, when it comes to the upgrade, there are several options:
1. there is no comment at all or so little that it does not make things any clearer
2. the irrelevant and trivial parts are fully commented
3. there are so many confusing comments that it is faster to read and understand the sources
4. the comments are wrong (say something else than the code does)

Mixture of the above occurs, but case 5. (comments useful) virtually never.

JW