Whats the harm? At most, people can encrypt things with your public key and then...?
I'd say that at most it forces you a bit more (if that was necessary) to check the SSH fingerprint of the machines you're SSHing into.
For example they can identify my different accounts, when I sue the same key.
Last time I tried to use the same public key for a second account, GitHub refused.
I also view this as information leakage. I keep some of my online pseudonyms completely separated, and stuff like that allows people to link them together, if I was not careful enough to use a separate ssh key.