I think one reason (however illogical it may be) that some product owners implement last cookie is to encourage a "mailing war". They want JVs to think, oh, everyone else is cookie-ing a bunch of people, I better one-up them by mailing. Oh, they mailed again, I better mail again too to "steal" some leads from them.

Of course, in this case, the product owner is really thinking about him or herself and not his or her JVs.