It helps to use the payment system. 9 payments of $12 is easy to budget for vs. taking the hit on one month. As a point of comparison, I've paid $8 a month to Six Flags this year for a yearly "membership". Of course, the SF membership gets you into every single park...
There was a survey floating around a little while ago where they were testing the waters for things they could add on to make the season passes more worthwhile. I expect to see some kind of multi-property HFEC membership option come around sometime. Probably a lot of branson discounts, plus the ability to go to other HFEC properties like Talking Rocks or maybe even DW/WA?