When something new like that is only available in a certain area, I'd say it's probably being tested before offering it nationwide.
And omg, I would SO get that... Well... One thing I'd change on it or make an option of either/or is I'm not interested in White Water, so unlimited visits there don't appeal to me.... So give me an option to have unlimited visits to Dollywood in place of WW.
Like, seriously, I REALLY want Herschend to come out with a SDC/DW combo pass. It'd be very nice and my friends and I would use it a LOT. We got Six Flags memberships this last year because we were in both the Chicago area and the Atlanta area during the same road trip, so we were able to go to both local Six Flags parks and already basically got our money's worth.
We live in Missouri, but do a lot of traveling, often through Tennessee, so a SDC/DW pass would be used a LOT.