Sorry, my reference was more along the lines of, depending on what Guards you play, or like, they could be radically different, ergo to me, there isn't quite the easy baseline for your average Guardsman. Yes, you could pick the Cadians, and I probably would, too, but they've got so many different flavors, different groups at different tech levels, that if a planet were found tomorrow, and returned into the Imperium after a millennium alone, and if their troops happened to have access to a cache of old jetpacks, and had happened to train in their use, it wouldn't be any weirder than lasguns, or Mongols with horses, or the Skitarii of the AdMech. There haven't been said Guards in the TT line, because Games Workshop doesn't like to crank out a million more different models, as it costs them money, and then we'd have to pay money, too. With there only being a million or so SMs in the galaxy, I think sometimes the IG have to be a bit more flexible, and have many more varied types of responses, as they are all they've got to fight the new alien, mutant, or heretic of the week. Saying they don't have jump-pack troops, just because they aren't featured in the minis line (now, you can certainly say that isn't what you intended, but that, books, and the occasional snippets from GW are really all this universe has for stuff, so I'm not sure what else you were referencing, otherwise) is akin to saying there are no female Guardsmen, since they discontinued the 4, or so, that I can remember them having. There's really no good way to know what all the Guard do and don't have; it's a question of what the author tossed in, or what the GM wants them to use against the enemy. Jump packs aren't SO high-tech (I'm gonna hear on this one), and if they can toss out Leman Russ, after Chimera, after Basilisk, a few jump packs shouldn't be too unreasonable.
Ah, I was referencing our guys in the Middle East, like Iraq, Afghanastan, etc. I always say a vague similarity between them and the Cadian minis.
IT isn't just about the miniatures. Yes, your right, there are various things there are not miniatures for the broad range that there are of Guardsmen, and there are many things that don't have models that I don't see as being outside the "feel" of the Guard. Trucks, or something similar, for one. Someone needs to move stuff about, but they don't make the most exciting tabletop unit. Bikes are another. There are also obviously the various visually distinct regiments that never get any miniatures.
However, there has generally been a fairly consistent general "feel" to the Guard (though I accept it has been watered down more recently) since the start of 2nd edition. 1st edition Guard were very different (in fact they even changed within 1st edition. They started out as the unwilling conscripts of the Imperium, though reliably xenophobic, and then become the best of the best of the Imperium's human forces). They were a much more punk (I am not sure if this is the best term, but it is the word I feel best encapsulates them) sci-fi army. However, when 2nd edition came along, they had changed, and there was much more emphasis on the… sledgehammer nature of the guard. They were not really a precise tool. They worked through the principle of staunch discipline (or maybe more "grit" in the case of regiments like the Catachans), manpower and lots and lots of firepower. Much of the feel (though I admit there were even variations then, but there was an overall sense) was a more comic book ww2 army than a sci-fi one. Jet packs are just not in keeping with that overall general feel, and personally, that is what the Guard are about to me (it is also why I am not so keen on the explosion of Valkyries that regiments seem to have since the plastic model came out).
In the case of the "mini-jump packs" in Guns of Tanith, I didn't think that much of it, but I could see an exception, as the planet being fought on had very special requirements, being one where many of the battles were essentially airborne (and it was Gaunt's Ghosts… which I don't really think does 40k very well). In my mind, such specialised equipment would be kept entirely for such specialised environments (and regiments), and they just wouldn't see use on a normal battlefield.
Those are essentially my reasons why I wouldn't have a jet-pack regiment as a "hit and run" regiment. I see bikes as being much more appropriate. Truthfully there is already a background description of a "hit and run" regiment. It was one of the Tallarn ones, and what they had instead were stripped down Chimeras and the like (they are also the element of the Tallarn's background which show the LRDG influence). Of course, in their case I do think they were fighting on a desert planet.
Oh, the Cadians. Oh, right. Well, yes they were obviously based on modern armies. Or at least the 2nd edition metal ones are, though with some hints back to the 1st edition Imperial Guard. The current plastics I think are just generic sci-fi soldier.