Just to play devil's advocate here, but personal messages could be used pre-sale in the event of license transfers (e.g. arranging the details of someone selling their license to someone else within the license terms). Other pre-sale personal messages can arise in arranging someone to install xF or find out the feasibility of other custom development before making the purchase, as the inability to do a particular thing might prevent that person from wishing to own a license.
On topic, I like the IPB system where the username is denoted by a red + for active customers, and I think it works much better than vB's system where you must simply post in the correct forum (because as mentioned before not everyone checks for "the correct forum"). To adapt the IPB system to xF, I think a badge (or whatever the things that are below your name in your posts are called) to denote active customers would be beneficial, perhaps only viewable by other active customers to address privacy concerns brought up above.