This question already has an answer here:
The usual decision when the host name and the name in the certificate don't match is to reject the certificate. I see no reason to desensitize users to Man-in-the-Middle attacks by telling them: "it's okay to accept a certificate although the names don't match".
I have Firefox Cert Patrol extension installed. Yes, I have read the alt names, but not everybody does that. Scaring a user away out of desire to save money on certs isn't a good tactic, IMHO.
EDIT #2: As reported by Carlos Campderrós (and observed by me now), the name mismatch is definitely encountered without any alternative names on https://stackoverflow.com.