The intrinsic benefit to holding as Steem in the form of Steem Power is that it earns interest like you said. You also gain more influence as a curator. For example if you have a lot of Steem Power, your upvotes and downvotes will carry more weight than a new user who has very little.
Market cap may increase if publishers and content creators with money realize that they can buy influential power on the network. Also, people tend to be greedy and like to speculate on price so sky's the limit on that one. That's the only reason why I'd purchase- if you think this site will get popular and market cap based on speculation goes up. If you're a conservative investor, I would just earn as much Steem by becoming a early adopter and use the site as much as you can.