And, honestly, I think this is probably the most common style of play out there. Pc's get a job (quest, hook, what have you), PC's undertake to resolve the job in a number of ways that they come up with on their own, job gets resolved, and it narrows back down to the next job.
Yeah, there might be three or four jobs lined up (again, replace "job" with hook or whatever term floats your boat) but, essentially, you have another choke point so the DM can prep the next session at the very least.
As the difficulty of "winging it" and the length of prep time increase for a given system, so does the tendency for the GM to push the players onto the prescribed adventure. Frankly, it's just more fun that way. Therefore, player character freedom can often be at least partially attributed to the system itself, insofar as it supports or restricts the GM's ability to provide multiple avenues of entertainment for the players. BD&D is emminently wingable -- it's just *that* easy to run and come up with stuff for. As you move along the AD&D->3E->Pathfinder track, it becomes less easy. I combat this (running PF now) by having a few different adventures in my game bag with some party/campaign specific notes. And if I throw out a random hook (as I am wont to do0 and the PCs bite hard (and I am totally unprepared) I have a few special encounters/mini-adventures ready to go. Since we only play for 3 or so hours at a clip, that'll cover me til next week and they're none the wiser (okay, they probably know, but they seem to enjoy it anyway).