to answer the question from the OP
Are MTB-specific flat pedal shoes really worth it?
if you are riding flat pedals, then yes
skate / bmx shoes and running shoes do not cut it for mountain biking
a big problem with using running shoes for riding on flat pedals is that the canted forwards wedge puts you in a terrible position for riding (I speak from experience as someone who runs several times a week using proper running shoes)
with flat pedal compatible shoes, we are not talking about riding flat pedals to the shops for beer, or dirt jumps or sessions in the local skatepark but proper mountain biking on the dirt going up, along and down the trail, especially in these conditions..
a decent pair of 5.10 or similar mountain bike specific flat pedal shoes, give:- pedal traction / grip, pedalling efficiency, foot support, ongoing wet weather durability and foot protection that even Pro model BMX shoes can only dream of…
worth every $$ and yes, I have owned and extensively ridden many skate / bmx shoes (Vans, Etnies, Orchid, Osiris, Airwalk, etc.) since the early 80s on BMX and MTB before finding the awesome 5.10 shoes to be to my liking! I also use clipless Shimano SPD and SPD-SL pedals for other applications, in comparison