I never delete code - only comment them

This is f-ing terrible. Put your code into version control and stop having it be bad, for goodness sake.

I've seen one reason to keep old code in: if the new implementation is complicated and the old one is short and simple, keep the old one as documentation for how the new one <i>essentially</i> works.

There are times when this is required. For instance, where I find there is a line of code that shouldn't be called. Commenting it out and explaining that it breaks so and so is preferable to deleting it all together because there is a risk another developer will make the same error too.

Sorry I don't agree with this. I delete code all the time. The difference is, when I write elegant code that I am proud of I save it. The majority of code I write is utility not elegant by comparison. This is like my art, or any art.