Future-proofing, mostly. The IANA has already run out of IPv4 addresses to allocate, so the address exhaustion will now trickle down to the regional allocators, then the ISPs, etc. Eventually, a player will try to connect to your MUD via IPv6, and if you haven't added support they won't be able to.
For me it's a learning activity. I wanted to play with accessing things on the v6 network while my ISP does not provide native v6. I started with a SixXS provided tunnel with AICCU on a single PC within my home LAN (ick…). I have since graduated to utilizing both a SixXS and HE.net tunnel on my router's WAN port that give me two routed /48's to play with inside my LAN and let traffic take the path of least resistance out of either tunnel. With (almost) native v6 on my LAN now I felt like attempting to extend my v6 use to one of my hobbies - MUDs :)
Basically, it won't matter at all for several years at least. Even though they've "run out" of IPv4 addresses, it will likely be quite a while before two players try to log in from different IPv6 addresses that both map to the same IPv4 address, since currently everyone who isn't directly on a backbone is being routed through IPv4 anyways.
Curse the people in charge for not making this switch mandatory with a drop-dead date for IPv4 support. Because of that, it'll still take years for all the ISP's to properly support IPv6. :(
That said, if you can get the two protocols to live together in your driver, it can't hurt. :)