There was a time one could walk a stream’s edge in Oregon and find Whitefish laying atop rocks, dead and wasting. They were and are still viewed by many as a member of the sucker family and that is enough to sentence them to a high and dry status.
The incidental catch of Mountain Whitefish (Prosopium williamsoni) has never bothered me. I love catching all fish, be they Trout, Whitefish, Carp or the lowly Chum Salmon, to name but a few. The Whitefish is often described as a fish of 8-12″ on average, but they can get considerably bigger in Western Waters (I have caught them in excess of 24″ on the Metolius River). The Whitefish has a range from the NW Territories down into the Western U.S. The fish readily take a small nymph and on occasion a small dry fly can bring them up out of the riffles and quieter water adjacent to riffles. Some say they hold in different water than a Trout.
The one place I do see this is in fairly fast riffles of say a foot deep with no structure to provide relief from the current. The Whitefish will be in that water and you don’t see a Trout attempting to hold in that sort of water very often (unless Summer time low water and depleted oxygen levels force them into that water for brief stints of oxygenation).
If along the way, you have to pass the time catching some Whitefish on an otherwise Trout barren day, they are worth the time of feeling the tug. Small flies, nymphs, are in order and never allow any one to toss them up on to the rocks. I believe most fly fishers are intelligent enough today to know that Mountain Whitefish are not a threat to their cousins, the Trout and are a sign of a healthy watershed in many instances.
SwittersB & Exploring 
We regularly find these in some of the smaller, faster waters in northwestern Montana. They are fun when nothing else is about and find them pretty routinely mixed in with trout in places. The will on occasion take a dry but are usually down in the dredging zone. I’ve found that when the trout hits the mid day lull they are pretty easy to target with something flashy, a rainbow warrior, PT flashback etc. They eat good too. The ol’ lemon, butter, pepper bake does them right.
As you mentioned, they do get some size to them and on a light rig in moving water they can be worthy adversaries. They do get to be pests when you know their are trout on the same lurk but the damn things pick your fly off before the trout get a shot at it.
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