Supposedly they bought them so they wouldn't go under. I'm hoping that's the case and it's just they see it as a good deal, they pay to keep, and dare I say, help improve OpenDNS going, and they get something that helps them (has security benefits for Cicso networking, at least allegedly that's the impetus behind Cisco wanting OpenDNS to not go under).
That reminds me, I think I switched DNS back to ISP's, there was some issue with Open DNS like a couple of years ago and forgot to switch back after. Not sure if I would've seen any benefit or not.