It's just a trademark, though, not a patent. My guess is that because the Dreamcast hasn't been sold for awhile, their hold over the name Dreamcast, as a trademark, might be disputed unless they renew it and state that they do still hold interest in the name Dreamcast and nobody else can have a thing (like they describe) called a Dreamcast.
I think the description (floor mat, etc) really has nothing to do with what Dreamcast was, but Sega just covering all the bases, so to speak, in case something comes up. If I had to hazard a guess, they're may be looking to lease their library to Microsoft for an emulator for Xbox 360 and offer games on Xbox Live. (Or the same for PS3.) And if there's an association of Dreamcast and Xbox (or whatever) they may want to keep another company from selling a "Dreamcast Dance Mat for Xbox" and cashing in on the name -- and confusing the hell out of everyone.
Another possibility is that they have on the books that the Dreamcast trademark has value to the company, and therefore an asset to be listed on their balance sheet. In reality it may not mean or be worth a cent, but they may be using it to inflate the bottom line value of the company.