Why is it disadvantageous to annotate books you own?

I can think of a lot of reasons why it would be advantageous such as being able to look back on what you wrote and what not.

As for disadvantages, I would say that if you wanted to give the book away or resell it you might have some problems with that since many people do not like writing in their books. Also you may damage the book over time with the ink. You might also regret writing an annotation and there's no easy way to erase it.