One reason I don't really fret the generous fatigue point (FP) rules in GURPS is personal experience.
At the moment I'm doing some MMA training in Japan, as well as visiting old friends - the experience blends, as almost all of my Japanese friends are my MMA training buddies from living here and past visits. I do this routine of doing every single class while I'm in town, travelling to friendly associated gyms and training there, hunting down old friends and the toughest-seeming new guys to train with. I push nonstop, make some Carousing rolls in between, and sleep little thanks to an early dawn and early rising neighbors where I stay.
This isn't the first time I've done this. My record is training the same day I got off the plane, and then training 12 days out of 14 (two Sundays messed that up) including at least one extra mid-day class. Going hard 4-5 days in a row on minimal sleep (5-6 hours, not my usual 7-8) while jet-lagged and doing almost every round of training isn't that hard. I'd do all of the rounds but sometimes partner switching means you really can't or lack of mat space means waiting. And I'm always here when it's brutally hot and humid except when there is a typhoon. Plus my carefully arranged diet goes out the window in favor of randomly selected foods and travel-ready portions.
It's a pace that burns the candle at both ends but generally gets bracketed by a short break before (albeit with travel stress) and a long one after. Not a big deal if you don't do it every single day for a long time.
The individual bouts of combative training and fine, too - I generally feel like I've got as much power at the end as the beginning.
Anyway, this informs my opinion on whether I should enforce long-term fatigue penalties or more strict FP recovery issues in my GURPS games.
My DF game is a case in point. Megadungeon delving is a lot like this in my campaign. The PCs are off for a while. Then they gear up and do a half to a full day off walking, fighting, sneaking, healing, running, dragging, climbing, searching, etc. with only snatches of rest here and there to make up for FP spent in battles or movement. And once FP are back, they're back - no Last Gasp or Long-Term Fatigue rules wanted, needed, or used. After a day of this, the PCs take their haul back to town, sell it off, and presumably get some rest before doing whatever they do in Stericksburg that pays for most of their upkeep.
Even the overland isn't that bad - travel for a few days by boat (Cold Fens) or by foot (Lost City of D'Abo), spend a day or two trashing the "dungeon," then repeat. You might be tired when you get there but not enough to really bother anything.
The fact that no one took Light Sleeper helps, too - no one is literally asking for points in return for special issues with travel and fighting and lack of sleep thrown together.
There is nothing wrong - and a lot right - with the rules I referenced above. They just aren't appropriate in my game.
I don't like to generalize from only my personal experience. But I do find that my personal experience provides a nice explanation for why the delvers in my game aren't slammed for many delves in a short time or intense delves in general. And why I don't fret cumulative effects of hard combat beyond that caused by direct injury. Here is where the generous rules do match my experience with doing this in the relatively short term - intense effort is possible even when tired, especially combative effort. Penalizing the PCs for it doesn't seem that fun, and would run counter to something I've done personally. So yeah, thanks to my own "vacation" plans ("Let's full-contact spar for 9 out of 10 3-minute rounds with 15 second breaks!") I don't worry about the PC's fatigue issues ("Let's kill orcs for 12 seconds and then hang out for 15 minutes resting and looting, then fight again in two hours.")