Also, not everyone plays a high gain amp, a lot of people have old amps, vintage style repros etc and, i feel this is the most important part, these people like the sound of a higher powered pickup pushing an amp into overdrive. Just like you don't need an OD or a fuzz pedal to get more gain out of an amp (remember this is what they were originally designed to do) but a lot of people use them because they provide a different flavour.
If everyone played high gain amps at full preamp gain the world would be a less varied an interesting place.
In terms of BKP's high gain PU's, seems to me that they're tailored not to turn into an over compressed indistinct mush like a lot of other manufacturers PU's do. Vintage style PU's, to me, seem to be tailored to reproduce the characteristics of vintage style sounds. The vintage hot pu's IMO are tailored to come out with that great hot rodded sounded that people got from rewinding vintage PU's back in the early 80's, which i think produce some of those the most open high gain crunches you can get. Not so high output to compress too much, but enough out put to add some more grit to the sound
Personally i can tell the difference between higher output and lower output pickups. if the gain settings are the same on the amp it's pretty clear (totally IMO). and you can definitely tell the difference in their dynamics.
So at the end of the day, it's all about your taste and requirements.
I myself am looking at new PU's for a guitar i have, and am considering using lower output PU's to get more of the guitars characteristics through, more dynamics and turning up the gain on the amp a little to compensate. But i don't have my pre amp gain anywhere near maxed, some people do. And that case, with an uber modern, ridicculous gain amp, PU choice isn't going to make an awful lot of difference because it's going to be all compressed to hell anyway. Which some people also like.