Back to the original question (I missed this thread until now), I think it varies on a case by case basis.
I didn't have one for about a year. I was fine without it but now that I have it I really do find it useful.
Here it is installed on my current tank. It's right around the corner from the tank, and across from our back door, where I let the dogs out every morning right after rolling out of bed.
So I'm walking by it anyway, and can immediately see my Tank Temp, Tank Ph, Conductivity, Caclium Reactor Ph, and Return Pump Flow. All without bending down, opening up the stand doors, and waiting for the P4 to cycle through it's screens with 2 parameters per screen. Same with when I come home. I open the door, walk in and immediately can see if anything is out of whack.
I also use it a lot to turn on/off a socket (like the refugium light if I'm cleaning it out), or enter feed mode or a maintenance mode. Again, easier than using the P4 directly.
I don't attempt to do any kind of programming/change settings on there. Too slow/cumbersome compared to using GCC.