I would much rather have one and not need it,than need it and not have it. I tend to run them about midway up.
The only problem is if I want to use a heavier OD/fuzz but not get the super in your face Rhonda Smith sound, I need to turn them off, which requires going behind the cab and also having a cab with otherwise good mids that reach up at least into the 4k or so range. One of these companies one day is going to realize this problem and just spend the $5 on adding a footswitch jack to the input panel to bring the tweeter in or out and make a killing being the only one doing it for a while. Using something like a ground control, you could easily have a tweeter switch as part of a preset. So for your bright funky Marcus Miller stuff, tweeter is on. Then you go into doom metal and it turns off the tweeter, turns on your sub octave and fuzz, etc...
So yeah, some of them sound less than great, but I have good cabs that have good sounding ones. But I have to usually just use a compromise setting in between bright cleans and low passed dirt. I'd probably keep them closer to the 3/4 range than 1/2 otherwise.