It is much better to have a car if you plan on staying in the park late.
Corey Sandler, in his Econoguide tour books (1990's vintage), mentions that if you missed the last shuttle, then you have to find your own transportation.
A recently published shuttle schedule (for Bonnet Creek) only says first come first served. What if people are already at the park bus platform and the last shuttle can't take them all?
Or at least you should take the stance that the only purpose of the last bus or train or whatever is for the people who missed the second to last bus or train or whatver.
Now when you are already at the park, just use Disney buses for park hopping.
On another forum someone mentioned $75. for a taxi from the Magic Kingdom loading area (TTC) to Bonnet Creek. That seems way out of line to me.