Of course, it's also a question of effort vs. reward.
For example, if someone needs to set up sock-puppet accounts and post a few innocuous posts before they can spam, will they really go to the trouble to do so? At some point you have to tightly tailor your attack to a given site, and it may not be worth bothering.
A number of the proposals I saw have this property--that is, it makes successful spamming too involved to be worthwhile. There's more leverage attacking widely deployed blog software than something custom that exists only at one site.