Now on a couple of levels I get it, mostly because the licensing agreement required a certain level of parody. Thing is, the majority of the "humor/parody" in the game is stuff that simply worked to begin with....like the idea of the Gnome Titan Groin Stomp or (again) the idea of Gnome Titan bars having an in-house weak poisoned drink called the Gutbuster. Getting kicked in the nads and (military) Grog Bowls are real world things....and players will undoubtedly interject far more humor into your game than what a game designer can write. The broader, rather silly, joke elements were usually, and easily discarded by the players and GMs......Gnome Titans were pretty rare.
....also, someone else in the larger gaming group was proud that they had managed to acquire some OOP KenzerCo HackMaster gaming books that still had their coupons!
Coupons were another joke/parody element that were largely pushed aside by the overwhelming majority of HackMaster players/groups. First off, the majority of the coupons were minor things easily hand-waived at the table. Things like having an extra healing potion or some generic piece of needed equipment....but there were some pretty good gems that were harder to find. The coupons had rarity levels!
They also had GM coupons and there was a coupon book. The GM coupons also ranged from relatively benign to straight-up "Killer GM". GMs were encouraged to retaliate in kind to coupon use. Players use a coupon that essentially negates a PC death.....the GM can, assuming they have one on hand, play "He's Dead Jim", which instantly kills a PC....GM's choice. Oh yeah, just the presence of such a coupon generally keeps things civil amongst a group. To me the real "joke" is that KenzerCo made/sold a product that gamers were eager to buy, but never use!
We still used the occasional coupon, and if you weren't a dick about it, neither was the GM. If you had a killer GM then you're problem wasn't coupon use, but general play.
While I had a metric butt-load of coupons, both GM and Player, I recall using one....one coupon in seven or eight years of play. I had traveled across the country for work and managed to drive a couple hours to play in a some friends' home group. I brought my DS Invoker and during the course of play I played my one coupon. I don't recall the name, but the effect was I got to cast one 1st Level Spell for free (no cost for components). This is generally a low-power throwaway coupon, especially since 1st level spells are cheap...except one: Find Familiar. Cost really wasn't an issue as it was only 1,000 GP. No, the real value in the coupon was that DS Invokers are prohibited from casting Conjuration magic, so this coupon let me break that rule. I lucked out and was able to get a "special" familiar of a Pseudo Dragon, which was perfect for my long-term goals for this PC.
I had considered using some other type of coupons in game, but I ended up instead going to a token system in my 5th Edition (HackMaster 5th edition, sorry not D&D). Players got a token whenever they did something that I thought, as the GM, was really cool, and each player got a token at the end of every game BUT they were not allowed to keep that token. Instead they had to award the token to another player and tell the group why. Basically it was akin to having a table MVP, which was a thing for 4th Edition. Players playing their class and alignment was easy enough to manage through the Honor system, but I liked the idea that players could earn a bit of advantage to "break"....more like "bend" they system. I do not remember the exchange rate, so I might be a bit off, but players could use one token for a +1 die bump, exchange 10 tokens for a re-roll/mulligan, 15 to force a GM re-roll, and each token was worth something like 10 XP per level. I hated it when a player was *just* shy of leveling with the rest of the group.
I think if I was running a group again, I'd tweak and implement a new token system.....