If salmon pads work better than ordinary black pads, any reason to ever use anything but the salmons?

Assuming not, then whats the point of dual compound pads? Seems to me that good pad (salmon) combined with bad pad (black) would add up to less than just pure salmon pad, right? So no benefit there?

Been using all salmon pads myself...

Depends on what the conditions are in which the pads are used. Wear rate and general pad behaviour cannot be summarised as simply as "good" and "bad".

What conditions do you generally ride in? i.e. is there much salt / sand / sun / rain / cold where you are?

Kool Stop's black pads aren't "bad", but they are intended for mostly dry conditions and wear better than the salmons. The salmon pads are superior in the wet but wear faster. I use salmon pads exclusively and see no benefit to the dual compound ones myself.

An attempt at the best of both worlds.....wet and dry conditions......does neither one well.

Marketing gimmick... Choose which one you want, black or salmon. Both makes no sense.

As others have pointed out, the KS salmon pads work better in the wet, but at a cost of shorter life. That's the trade off, wear rate vs. wet braking. They also may not be suited to the hot weather and long descents, one may find out west.

As such the dual compound only makes sense for folks who almost exclusively ride in hot dry conditions, but want a measure of improved braking in case they get caught in the rain.

you should see the BBB tristop pads. Per the name, they use 3 different compounds on the same pad.

Yes. A blended pad is more marketing sizzle than anything useful. Best idea would be to buy pads suited to the riding conditions. And if a change in conditions is anticipated then fit the appropriate pads.
As might be done with tires.

Modern slip-in pad holders for both road and V-brakes (and some canti's) make this quite easy as no realignment is needed after the change. Just remove a small bolt or cotter pin, slip the old pad out, the new pad in and replace the fastener.

There are times you just want to slow down or stop, then there are times you need to stop as fast as possible. For those times, I use salmon exclusively. As long as the brakes don't wear out in 1,000 or 4,000 miles, I don't care. What I see now, I'm going to get 20,000 miles or more out of salmons. I have 10,000 miles on them now and they look great.

I actually have a backhanded compliment for the dual compound.
My 2008 Specialized Globe had too good of front brakes to the point of doing nose stands if one wasn't careful.
The dual compound pads have now given me a controllable front brake.

If I remember correctly the KoolStop Dual Compound pads were a 60/40 split with the salmon part being 60% and on the leading edge (at least on the inserts I used). So the black part handled the highest loads with proper pad toe-in and helped reduce wear on the salmon part. Extra lever effort in wet conditions probably put more load on the whole pad loading the salmon part more fully but the effect was pretty seamless to me. Picking a pad suitable for the precise riding conditions might be ideal but I never considered these a 'gimmick' myself.

you can get mixed metallic sintered and organic friction material, disc brake pads, too.

These contain mixed friction materials blended together but the pad is the same composition across it entire surface, not divided into segments like the pads being discussed above. Not an analogous sutuation.

The first compound squeegees away water, the second compound gives good braking, and the third compound gives a close shave...

(there are triple compound pads as well)

Nope they have bands of the different materials on the disc pads too ..
Can't be specific, the wholesalers catalog is at the shop, I'm Not.

I thought the black part was to cut down on squeal... nevertheless, the dual-compounds work so well on my bike that I have no complaints.