PAMBA went through this exact process about 2 years ago. In the end, we decided that the biggest benefit would be having the 501c3 status, but that alone was not worth the extra cost/overhead/what have you to become an IMBA chapter. We applied for our own 501c3 status earlier this year. The biggest downside of applying for your own 501c3 is that it takes time. It has been almost 1 full year, and we still have not had our application processed by the govt. (darn election year)

If I had to sum up my own conclusions on the subject, it is this:
If you are a small club, struggling to get off the ground or attract new members, then this will benefit you. It will give you a huge boost to get your club up and running.
If you are a well established club with a substantial member base already, then you will find little new benefit (other than knowing you are supporting the greater cause of MTB) but you will find more of your $$ going to support IMBA. I would say FORC is definitely in the second camp. I would also speculate that in the end, I doubt it will make much impact to your club or its members, either positive or negative. I believe most people don't even care (myself included) they just want to ride their bikes and support the local club.