My feeling is that (paraphrasing here) "we might get blocked occasionally" and "we need this many IPs on our MTAs because they can't handle the load" are *not* legitimate reasons for requesting so many addresses.
It is definitely not your job to help spammers evade blocking. If someone's blocking their mail, that's a message to stop sending it, not to try to sneak it in the back door. The valid scenarios for spreading out IPs are so rare (and generally involve guys with guns) that you can ignore them. Legitimate bulk senders want their IPs in a compact block so they can set up feedback loops from ISPs and stop sending mail that people don't want. As other people have noted, you can send vast amounts of mail from a small number of IPs, and anyone big enough to have a valid need for a lot of address space is also big enough that you have already heard of them. Friendly threat: around here, if we know that an ISP is hands out IP ranges for snowshoe spamming, we often block their entire address range preemptively to avoid the tedium of blocking it one little chunk at a time. R's, John