It can be very useful for incorporating solids into a parti pedigree but genrally not vice-versa as you run the risk of mismarks. But when you breed a solid to a parti, then go back to the partis with the succeeding generations you can get some very nice dogs.
However a parti can produce some surprises in a solid pedigree, as when a friend of mine bred solid to solid and got red, black and blue roan!
The parti was 5 generations back!
I suppose it depends on the breeder and the purpose for the mating. As said, workers are bred with no thought to colour, and puppy farms, back yard breeders and those breeding for the pet trade will also cross colours. But most exhibitors, serious show breeders do it very sparingly and very carefully.
That said, if someone were just looking for a pet or worker, not a breeding or showing prospect, you can get a perfectly fine puppy if all the other things (health testing, etc.) are in place. There is nothing wrong with a mismark, they just aren't
desirable in the show ring or whelping box