I'm always a little amused by folks who don't make ADRs because "...we don't want our dinner reservations to rule our vacation!"
We make dinner ADRs for every night we're going to be eating at WDW. If we change our minds, we just call and cancel. You don't even have to do that (You can just not show up), but we always call so someone else can get the ADR, rather than standing in line.
Several times when we've visited MK, we have seen signs at Town Hall saying there was no TS availability for the entire park, at any TS restaurant, for the whole day. And we've also seen a number of irate visitors who didn't do their homework and didn't know they needed to make ADRs.
During December, we have also seen situations where some Epcot restaurants (Le Cellier, in particular) were not even allowing walk-ups because they were so overbooked with CP packages they had no hope of accommodating anyone without ADRs.
To me, ADRs are just an easy way to get peace of mind about dining where we want...and still know that we can change our minds at the last minute if our plans change.