Does this actually contribute any flavor or aroma?
All I have heard is that it is a waste of hops. I have never tried it myself.
Someday I intend to find out for sure. My bet is that if you used noble hops, the flavor and aroma might carry though the best. I intend to try it with a lager. Like maybe a nice Kolsch. 
I have done it for Janet’s Brown. That beer is moderately hopped, so it’s not the best recipe to tell. I like the way it makes the garage smell during the mash.
Not IMO. I did it several times when it started showing up in recipes and found no flavor or aroma carry over. IIRC I did it the first time I brewed Janet’s Brown - I agree that’s not the best one to eval. I did it on other beers however that were much more lightly hopped and got nothing. Does make the mash smell nice, though.
I’ve done it quite a few times and found it inconsistent. Sometimes the flavor and aromas really come through while other times not so much. Mostly I expiremented with Pilsners and Saaz hops.
In my experience, none at all.
I tried this with a Koelsch once. I used 6 ozs of Sorachi Ace in the mash. Bittered to about 15ibus during the boil. I detected nothing of the Sorachi Ace, which is a pretty distinct hop. The mash did smell nice though.
If mash hopping has no effect then what is different about FWH that makes its presence known in the beer?
My guess is that since alpha acids are relatively insoluble, most are left behind in the mash tun. The ones that make it through to the boil can then be isomerized. That’s just a WAG, though. If that’s the case, then a decocted version of the same beer should end up at a higher IBU than non-decocted.
Edit - that may be why it seems that we hear mash hopping in reference to German brewing? Again, just a WAG there.
I tried it on an early Pliny recipe made with a neighbor. Who knows if it did anything, but I have never tried it since. My mashes are simply water and grains with some tweaks to the water and an occasional adjunct addition. Hops are for later. If I knew what I would get out of a mash hop, I would consider it, but until someone gives measurements in terms of IBU’s or flavor or aroma, I won’t waste the money.
Sounds like it could be called “Homeopathic Hopping”
I mash hop when I make a big American IPA and I want to clean some homegrown Cascades out of my freezer.
I use so many other additions that I have no idea whether it is effective. But, as others noted, it smells nice.
The fact that first wort hops stay in the boil for one. Mash hops stay with the spent grain. Mash hops don’t have no effect, but from what I recall of the 2014 NHC presentation, they only show a 15-20% utilization compared to a 60 minute boil addition.
As I mash in my modified Janet’s Brown, I started to wonder if mash hops can lower mash pH. No pH meter to test, just a thought.
If you mean due to alpha acids, everything I’ve read says that there is so little acid it has no effect on pH.
I’m going to say no, or not enough to make any difference. I measure pH like a lot lately. The pH shift between preboil and post boil doesn’t seem to be corollary to hops. On one occasion I get a .2 pH drop during the boil, usually little to no drop. I think that first noticed .2 pH drop may have been operator error. Anyway, results seem to be fairly the same regardless of hopping rates or AAUs. So if 4 extra ounces of simcoe/mosaic dont change the boil pH, I doubt that a couple ounces of hops changes the mash pH.
Think about it this way. Supposedly, .15ml of 88% Lactic per pound of grain bill will drop .1 pH. So in a 10 pound grain bill thats 1.5 ml to drop .1 pH. Lactic is a weak acid, i assume that hop acids are too. But 88% is quit a strong concentration compared to 4-15%, not to mention utilization… even at a boil, hop acids are not being utilized anywhere near how the 88% lactic is being utilized. I dont have a clue how readily hop acids give up hydrogen ions, but I have a feeling its not on the same level as lactic. It seems too that a lot of us who add hops at whirlpool in ~170F for 30 minutes aren’t detecting any additional bitterness from the acids. At a <160F mash there probably is near zero utilization.