We've got spare mitigation/capacity in Dallas if you're interested (through Psychz Networks). Layer 7 mitigation included, and we can meet your requirements for BGP.
I hate Spamhaus, only one previous listing, and they've listed 3 of our ranges. Absolute joke of a company, can't even be bothered to respond to our emails.
This is one of the things I stress to first-time colocation customers. Going with a colocation company that also offers dedicated is good for this reason.
Fairly sure it's just about flexibility, as many of us know ARIN have some pretty strict policy about justification for blocks already in-use in your network before assigning new resources.