simple really: I prefer shooting from the shoulder.
A number of others: my use is recreational and so there is absolutely no concern about the stock taking up too much space.
The fact that H&K made the thing with a "lanyard" hole at both the bottom and top of the grip seemed to suggest it was designed to accomodate some sort of attachment (that's how i plan to attach mine).
While i don't plan to shoot it much, i do plan to shoot it more than once a session, if i have three or four ammo loops on the stock i don't have to keep track of where they are.
Regarding the 26.5: it is less expensive, at least initially, than the 37/40mm and will adapt to those launchers should i decide to go that route.