A 3-position switch has always worked for me for a couple of reasons.

I've never wanted anything more than a takeoff position and a landing position. OK, I have also wanted an OFF position. So 3 positions have been adequate for use. It's not difficult to figure out a starting position for the two flight modes. And then it's dead simple to test if they're good positions. When you're testing, you know absolutely what the deflection is on the airplane, so if it isn't good enough, which way, and about how much, to change it on the airplane.

It's also very easy to visually check the switch on the TX to see that the flaps are up. And you can also check by hand without taking your eyes off the airplane to see if you've got landing set or takeoff or OFF. With a dial, that ain't so easy to do without taking your eyes off the airplane.

I found out flying gliders that screwing around with infinite flap positions is just that, screwing around. I wound up botching too many landings from diddling with that dial instead of flying the airplane. With a switch, while I'm flying the airplane, I flip the sucker, and keep on flying the airplane. I know exactly where the flaps went to, and know what to expect from the airplane. With the dial, I was constantly trying to guess if I'd deployed enough or too much or too little and trying to figure if the plane was flying like it was because I'd turned the dial too much or too little or if it was the wind doing all that to the airplane or if I had too much elevator or if ....... thunk!

With a switch that never happened. Flipped it and flew on in........