Yeah, pretty much. Especially if you have certain times in mind for when you like to eat. Walk ups can be had, but you might lose a lot of time waiting. If you call that night for a table, the times might be very limited (do you want to dine at 9:30 pm?), and some places will be completely booked (breakfst at the castle). A slow time like that you might have some better luck, but you never know.
Dining at table service restaurants has *definitely* gotten harder to come by. In the past, we would book some things in advance, but we would add other things once we got there or do occasional walk-ups. That just doesn't seem as feasible anymore, even at less crowded times of year. I've heard restaurants say they aren't taking any walk-ups at all or that the only ADRs they have available are for 9:30 at night.
I don't think it is a must for every meal during slower times. But for character meals and other restaurants you REALLY want to visit, make ADRs to minimize disappointment. We made no ADRs on our recent visit in late June and were fine, but we are flexible with regards to meal times and our kids are older now and can easily withstand a later (or earlier) dining time. We also visit WDW twice a year, so if we miss out on a meal, we know we will have other opportunities. Some places will always be very popular; ADRs are a great way to assure you will not have problems. And making ADRs ahead of time gives you one less issue to worry about!
What it is is that regardless of the time of year, the more popular restaurants will "book up" first. Only after they start booking up do some of the less popular restaurants start getting booked. So during the more crowded times of the year, more restaurants will run out of ADRs to offer, but the more popular restaurants will run out of ADRs to offer all year long. While you may be able to get 3:30 PM reservations at some of them at the last minute, you're not likely to be able to walk-in to any of them for dinner at dinnertime.
Yes! Unless you do not care if you eat in a TS. If you don't have ADR's then they could turn you away. I have seen this happen far to many times. Better to have them then to not.