I wouldn't consider the want/need for flexibility entitlement. And if you deem that entitlement you must also deem the individuals wanting predictability entitlement as well. It's a privilege to access the park.

I've been on trips where the weather has drastically changed from the original forecast forcing me to stop early on a lake I did not intend staying on. I believe booking by campsite can be feasible if it's within a half days paddle of an access point. But for the deeper interior it should be left as is for safety reasons. Like stated above, if you can manage to make it through longer portages, I don't think you have mobility issues and need to book site specific. I do believe access points and lakes within short portages such as: Tom Thompson, teepee, little Joe, ragged, pen, North tea, etc. could potentially be booked site specific. But interior shouldn't change. Opeongo would be a good lake to have the site specific system in as you can get a boat taxi to that specific site, or close to it. 

Wind bound, gear issues, unpredictable weather, forest fires, food problems, etc. All variables that can change the original travel plan. So switch it to site specific and now what are you supposed to do in these situations? Not everyone has or can afford  satellite communication devices to relay messages to have someone at home change the itinerary.

Plus, when staying on a new lake, half of the fun is searching for the "best" campsite.

Thanks for your insight!