I walk over to BTMRR with my FP+ and have no wait. I do not have to figure out what to do with my wait time - I have none.
On the other hand, you walk all the way over to BTMRR and then do what while you wait after grabbing a FP-? Get on a lengthy line at Splash Mountain? I'd rather have a FP+ in my hand for Splash as well - no wait.
If you can grab a FP-, but then have to wait or linger for your to return time, then how does this prevent wasting time or the running around that you mentioned was a burden?
Lastly, if you are at Magic Kingdom during slow days, as you've described, then are there that many rides that would require FP- for adults with no children that would be missed without a FP? If Slash has a 15 minute wait, Pirates and Haunted Mansion wait times would be even less; no wait time burden.
I'll tell you our FP+ rides: Splash, BTMRR, Belle, Ariel, Buzz, and Meet & Greet (soon to be 5 y.o. daughter). If my family arrives in the morning, we can ride/see half of these without a need for FP of any sort, then other half by utilizing FP+. On the other hand, the FP- method came with far more uncertainty and plenty of more running around.
Hey, I get that people who were able to grab FP- at-will have a sense of loss with FP+. And as you mentioned, people with small children are ever so grateful for the ability to have a FP+ in-hand before getting out of bed to attend rope drop in order to make certain that the little ones can ride their favorite rides without having to run around or wait stand-by for an hour. But your observation about planning to the minute is without merit when FP+ return times are a one hour window AND you know that window of opportunity in advance. And if you do not want to schedule FP+ in advance, you can grab what amount to FP- at the kiosks - yet another way to limit running around.
I wonder, do you have a problem with being on-time to ADR's? After all, there is a smaller window for return times. How is this not a burden to your touring plan?