This has bugged me a bit, too, mainly since I started using smartphones. All of my Windows Mobile devices only could have one mobile number per name which made little sense to me since I'm not the only person who knows people with multiple mobile numbers. But then again, Outlook natively can only have one mobile number and I am betting it has to do with Microsoft Exchange only allowing one mobile number per name, and since Outlook is an Exchange client, it carries the same limits. Considering that it seems that BlackBerry Enterprise Server is most at home with Exchange servers, and of course the vast majority of BlackBerries in use go through a BES, it does make sense that if Exchange only allows for one mobile number per name, then the BlackBerry should carry over that limit, even if that limit doesn't make much sense nowadays.
BlackBerry PIN: 204CE92F
Google Talk: donovan1983