My guess that the people who experience changes by changing to inclines is something like this:
1. They switch to the incline press.
2. They also sometimes have a new zeal for training, thinking they're gonna start making a good change in their chest.
3. They bring their incline press up by 50+ lbs.
4. They say, "wow my upper chest is bigger"
I wonder if they realize that it's still a press. If they added 50 lbs to their bench, their chest would have grown everywhere too (including the upper and inner areas).
HOWEVER, there is a clavicular head. An incline press may target this area a bit more. This would account for some extra growth.
Here is the big HOWEVER though, you need to concentrate on increasing the load (whether it is over time with strength training or in cycles by HST) to see REAL CHANGES in your pecs.
It's really not a big deal to have both presses in a workout, though. I myself do a set of hammer machine presses as well as a set of barbell inclines.
A PRESS IS STILL A PRESS. Here is where many people make the mistake. They want to just add the exercise into their routine. If your current chest routine is 3 sets of a bench press (and believe me, you don't need any more than that for hypertrophy), you should not add another 3 sets of inclines. Replace a set of flat with a set of incline, and be done with it.
This post was kind of long and rambling so I'll cut it short here
