Because it upsets people like you.

It's also the way I personally choose to protest the corporate and government surveillance state we're living in.

Why is it damaging?

It actually isn't. Sometimes data has too much purity. Noise can actually help.

db poisoning can help, but just having an account (10 is even worse) inflates their numbers and strengthens the perception that people should use gmail.

A long time ago, before the PC, market share perceptions lead to the idea that "Nobody got fired for buying IBM". It was a very effective trump card a salesman could use with managers to scare them away from the competition. In the PC era, "Nobody got fired for buying Microsoft" was abused in the same way.

This is really just a consequence of Metcalfe's Law-style network effects, which is why it is usually a bad idea to use anything that is known to be a problem; even if you use it maliciously, just the fact that someone gets to legitimately claims they have a larger clientèle becomes an additional barrier in the path of any competitor.