Admittedly, I buy stuff that I don't need. After all, I am about to buy a frame and build another bike when I have four in my basement. I like hydration packs, I have had a couple in my day, it has never occurred to me that I need something to tell me how much water I have left. If you are concerned about how much water to carry, fill it to maximum capacity, so what if you carry a little extra weight. I have run out of water on hot days, a device telling me I have 0 ounces of water left wouldn't fix the problem in the middle of nowhere. I certainly have never needed a device to let me know that I am thirsty (hydration goal feature). I am not sure why someone would even want to spend $20 on a device that requires yet another battery.
I think a good product would be a "gunk" meter to tell me when that nice layer of algae is growing on the inside of the reservoir or in the hose.