It's a fine (if boring) addition to the game, nothing wrong with people making their own wolves prettier with it.I don't use it and imagine I never would, just because if I have 4 color slots to work with, you bet I want to use them all. But I get it, WW-only dens can't do breeding projects and what not.
I do think people intentionally being dishonest about its use-- selling a wolf via its image without linking to it, hoping prospective buyers won't go into their den to find it and check-- are a fairly rare thing, but it's always worth thinking about!
Always double check! It's not limited to Plain, a lot of people will use the fawn accent in a similar way. There's been quite a few times I've seen a perfect project dog, double checked it, and noticed a hideous accent hidden by fawn's dinky little horns.
Most of the time, I think people aren't necessarily trying to deceive you, maybe they are perfectly satisfied with their breeding pairs and don't imagine that anyone would mind a hidden ugly color. But it does mean I wish we were able to hover over a dog on the flea market and see its colors (and maybe sex, because puppies share poses) to make checking easier as a buyer. :P