I prefer different manufacturers when possible. I've gotten bad lots and ended up with hard drives that all fail very close in time frame to each other. I think that's what happened with my RAID5 that blew a drive and then lost another drive trying to rebuild the array to a hot spare.
I'm a little OCD about some things but keeping my hard drives the same isn't one of them.
I have seen this too, but I still prefer to stick to one brand, model, and size. But then I don't touch raid any more unless it is in my work environment, and when you have 3 to 24 drives in an array, mixing and matching seems to be a bad idea.
In my personal PC's I no longer care about the minor performance bump, particularly since SSD's became mainstream, and personal computers just get heavier with redundant drives.