Every campsite and cabin at FW is on a loop. When you walk to the end of loop there is a bus stop nearby. There are 3 bus routes within FW that only run back and forth to the Outpost (main bus stop) and the boat dock. One of those 3 routes (purple, yellow, or orange) is always going to go past your loop.
Essential? No. You can always walk to the end your local loop and take a FW bus to the other transportation. Some of the cabins are close enough to the Outpost that some people walk the extra quarter mile or so.
Depending on how close we are to the pool, my wife still likes to rent one. She also likes taking it straight to the main transportation and having it ready to go back to the RV when we return. Most of the time I would skip it, but once we have it I wind up using it if I'm going to see a camper in a different loop, or want to drive through the other loops and look at other RVs. Most days it's a luxury. Taxi would be cheaper for the distance If you're only using it to make it easier to get to the parks, you're saving yourself one short connecting bus ride each way and up to 10 minutes or so as you wait on a connecting bus.
3 months out I would be very surprised if you can not get a reservation. I've always booked it on arrival. I picked one up on Jan 2nd and they were pretty tight, but they had a cart available.
Also remember that you pick up your golf cart after 1pm, so If you know Day One is a day you're going to be in the parks early, you would want to wait until you were going to be at the cabin in the afternoon to schedule your rental.
Others have used outside rentals, but at my last research, they don't let local companies deliver within the park any more. Last time I researched it, you needed to be able to tow a trailer from a mile away, drop the cart inside FW, then return trailer and drive back to FW. Maybe someone has a new plan.
The carts are well serviced, have charger built-in, and only take seconds to turn in your your final morning. Also remember licensed drivers only and they want all licensed drivers to be there at check in to sign rules list etc. Probably I've forgotten to take my wife every time
Hope this helps.