I just don't like them! There is so much richness lost with flats, the over tones that let tube amps bark, and effects speak much better with rounds. I can still mute and thump with rounds for the rare songs that really need such a sound.
I still have one bass left with flats, and its a P bass, but I dont prefer playing P's either...its only there because I'm a Jamerson fan. But I'm always wishing I brought something else whenever I do take it out of the house.
Ive always had duplicates of basses, and 15-20 years ago I used to keep flats on the 2nd of each style bass, but realized I was never reaching for them.
Ive tried lots and lot$$$ of brands of them, and Dunlops being the only ones I stuck with because the tension is most similar to rounds.