I think most people mix PRL lines to enhance the traits they're looking for ie. nishiki has really nice darker red bodies and legs, xin and feather have brighter bodies and not much leg coloring, ....all of those are from my experiences and from what I can tell when I look at the PRL that I own with those lineage in them. I can't say much for Ellen Wang, Benibachi, Ebiten, Crimson, Hakata, Elegant Taiwan Lady or any others since I don't own them.
I mix my PRL so that I can try and enhance overall leg coverage and try to get deeper red in the next generations but I have a bunch of lineages that I keep in one tank so it's hard to tell who is the father of the soon to be babies haha. But once I find out which babies have the traits I want it's culling time LOL
I know of some people who keep one line of PRL only tanks and work on those PRL for the traits they want to bring out in them too. So all in all it's going to be a preference to what you desire and what goal you want from them hehe
From what I've read if you buy the super high end PRL ($100+) the offspring of those PRL aren't worth that price tag because they weren't breed in the original farms tanks and are considered "made in the USA" PRL so technically they can't be called "(insert shrimp farm name) PRL" anymore but you can say they're offspring and breed in your tank though!