I am currently maintaining a few different identities each associated with different gpg keys (for personal, academic, professional correspondence etc., whether or not this is a good idea is probably a matter for another question). For convenience, I would like to use the same (strong) password across these three keys. I think this is OK for the following reasons:
These keys are all subkeys of master keys that are kept offline in a secure location. They can all be revoked quickly if necessary.
All these subkeys are stored together (I need all of them on all of my devices). As a result, if one is compromised all of them are compromised, and I would revoke all of them (and use a different password for the next generation of keys).
Having said this I am no expert. Enlightenment one way or the other would be much appreciated.
OK password reuse has some obvious implications. For instance if one is compromised, they are all compromised. As per the comments, more interesting to see would be if there are non intuitive implications. For instance: Is the password easier to crack if there are 3 keys using vs just the one.