First of all, a partial hook-up is water and electric only. If you are camping in a tent or a pop-up there is probably no need to spend the extra money for a site that has a sewer connection.
I just stayed at ASMu the first of this month and I, personally, really missed staying at Ft. Wilderness. We usually go for 2 weeks with our camper and I just love the fact that I can sit outside in my pj's with a paper and my coffee and just watch the world go by.
It IS laid back and, I think, a real hidden secret at WDW. There is nothing like spending the morning at one of the parks and then hanging out in the serene quiet of Ft. Wilderness for a couple hours before braving the crowds at the park once more.
We rarely rent any of the bikes, boats, etc. Just the last couple of visits we have rented the water sprites on Bay Lake and that was fun, but if you are short on $$$ something that you could live without. I found it just as much fun to ride on the ferry between the resorts. It is smaller and you get that laid-back "wind in your hair" sensation without spending a dime!
As far as perks for staying on site - YES!! You are a Disney Resort guest in every respect. Just like staying at the other resorts, you are entitled to the Extra Magic Hours and the free parking. And just like the other resorts, you are restricted from pool hopping. Ft. Wilderness guests can't use the Poly pool and Poly guests can't use the Ft. Wilderness pool!!!
If you are into camping and are a good planner/organizer, I think you will have a ball and a very unique experience!