unless I'm missing something, entirely ineffective.
The simple scenario is where a hacker gains access to an individual's Dwolla account and transfers funds to the hacker's own Mt. Gox account. If Mt. Gox has identify from verifying the account, then Dwolla would know who supposedly received the funds. This also lessens the chance of a fraudulent claim that the transfer was not authorized.
what would keep me from verifying the two differently, with different addresses, identities, etc?
I don't think it matters, at least not for deposits. I can send my Dwolla funds to your Mt. Gox account I believe. But with Dwolla being U.S. only, presumably only Mt. Gox accounts that are verified with a U.S. address then could receive Dwolla transfers.
But you can't withdraw your Mt. Gox USDs to my Dwolla I believe -- those two must be the same party, I believe.
So this verification cuts down on the permutations of risk to ones that are less exploitable.
