How many people that when they started to use pillows noticed a much greater improvment in their hand speed and fluidity compared to when they just used a pad or something with rebound? And did anybody end up with the bad habit of "playing into the drum".
Here is my take - based on what I know about the human body as it relates to kinesiology.
In theory, if you apply a stimulus more difficult to perform than the actual task, the body will respond with a heightened level of skill, which can be trained to maintain that level. An example is lifting weights. If you lift 20lbs, and your body is used to lifting 10lbs, it will need to adapt (recruit additional muscle fibers) to perform the skill - however at some point, your body will be able to respond to that 20lbs without recruiting additional fibers (this is the dreaded plateau as many people call it).
So given this theory, people have tried to recruit crazy "training" to super-charge their skills. An example, basketball players training for more "air" by jumping against resistance, football players/swimmers wearing weighted vests, etc. The idea is if they can improve their performance against a greater stimulus, they will be super-performing at in a normal scenario.
Let's take this concept to a baseball pitcher or QB. It isnt always about how hard you throw, but they rely on grip, angle, release, etc. There was a point in time (perhaps it is still going on in some circles), where these people would throw with a weighted ball - or execise with weights following teh same motion as throwing - the idea is they will be able to throw farther and faster when they get in the game.
The primary failure of this thought is when they train this manner, they are changing the way their muscles respond to throwing normally - and by changing this, they will not perform better in a normal circumstance - just differently. Some people, this difference can be tangible in what appears to be faster/harder, but control is sacrificed.
Now let's apply this to drums. The fact that someone is "drumming" is good - muscle memory in the hands, grip, timing, etc - but I dont know of a single drummer who just posted a picture to brag about their new Zidjian Pillow-top Cymbal!
For those who think hitting a pillow will help IMPROVE performance is seriously delusional. Take the time you spend hitting a pillow and work your kit and you will get the same, if not more benefit.
Lastly, don't discredit the act of sticking on any surface available. It's not about the surface - rather it's that youre practicing... and in the end, that' all that really matters in helping improve.