Because that way, anyone sniffing or monitoring your traffic doesn't even need to crack the systems to steal the hash, and so you'll be saving everyone some time?
Edit:
I'm trying to illustrate the general principle, that you shouldn't take any action thats visible outside your secure perimeter, that depends on knowledge of your password.
What you define as 'outside the perimeter' depends. In the case of your corporate systems, its probably everything outside the corporate network. In the case of your gmail password, its everything outside of [your computer, the SSL connection to google's auth servers, and those servers].
You shouldn't ever leak any information outside that perimeter, that reveals knowledge of your password.
Its generally pretty hard to steal the password hash; if you start revealing what your password hash is to someone doing passive analysis, you compromise a lot.
If its worth thinking about poisoning hashes to protect, then don't try and poison the hashes!
