Well for bettas, even if the PH itself isn't the best, simply having it stable is extremely more important. Rather it'd be stable than off.
Plus if you use it anyways, it reverts back to the original PH, making it kinda worse for bettas as well. (Not entirely sure about that, but I've heard multiple people say this.)
So I've heard. I don't use this, nor am I going to.
And you would be testing it just to see what it is too I guess.
Like if your PH was off, you could use a different water source with hopefully better PH. And when adding another fish, test their water and your water and see how long it will take for acclimation.
It's helpful to know your ph if you're adding water at all. For intance, right around the time I was thinking of changing my substrate over completely I was going to save half my water and add in new water for the other half. BUT when I tested my tap, the PH was 6 and my tank's PH is 7.4. I thought that was too big a shift, so I saved all my water. If you're chaning out a lot of water at a time you should know what your ph is in both the tank and your new source.
I've heard very mixed reviews. I want it for myself too actually, but i've never seen it in stores.... some worked, some haven't. I'd keep looking at online reviews and such. If you do buy it, keep me updated! I'm interested in it!
Oh, but I think the main flaw for me is that it takes several hours for it to actually get updated. Or something like that...