The way I can see a benefit here is for somebody who owns 2-3 businesses. If you have 3 business cards (each for a different business) and one personal card, you've got 20 nights/50. That's a solid head start...
Tying cards from multiple people to one SPG account is just asking for trouble. Their T&C may not explicitly prohibit it, but I'll bet there's a clause in there that gives them broad subjective discretion to address things they don't like happening.