Outside of Book 2 (meaning maneuver drives need a powerplant and huge amount of fuel)? No. But I'm not even sure that it was a design driven idea in the first place. I think it was a scenario driven design and that the whole complicated idea was the reason, not the result.
I think someone noticed that you could do a 100T jump-4 ship if you left out the maneuver drive and that the whole X-boat system arose from that. It seems more likely than someone saying, wouldn't it be neat if there was a system of courier ships that couldn't move on their own? After all, it's a pretty silly idea (IMNSHO). What scenario are you referring to? I'm not aware that there was an adventure involving X-boats until TD1.
I can see J5 because at least one link (in the Marches) is J5.
I can see that one link being a mistake (or, just possibly, an experiment) because the background text explicitly states that X-boats are jump-4.
The idea being that maybe Book 5 is best explained as new mature tech and Book 2 as old obsolete tech.
That doesn't work. The Imperium has had jump-4 technology for 700 years, jump-5 for 400, and jump-6 for 100. The AHLs were designed with the technology described in HG back before the Solomani Rim War.
So Book 5 allows replacing the J4 system with J5. Even if all the links don't use J5 it still gives you extra fuel in case of misjump or to speed up system pass through. You might even have systems that wouldn't need a Tender. And it adds a little versatility in times of war.
So does a jump-4 design with 10 T of extra fuel. And you're much more likely to not need a tender if the X-boat can maneuver on its own.
As for why not J6, because even with Book 5 it won't quite work in 100tons.
Really? Is that because it requires a 20T bridge? Because it looks to me like there would be room otherwise, but I could be wrong.
Then of course one wonders why the apparently arbitrary limit on making the X-Boat system limited to 100ton ships? (Again it goes back to Book 2 where you could get J4 in 100tons at TL9).
Whereas by HG you're not limited to a handful of standard designs and can custom-make 110T hulls if that's what it takes to make a jump-6 X-boat.
However, jump-5 and jump-6 X-boats will be more expensive than jump-4 X-boats, so I can totally see jump-4 X-boats being retained on the secondary routes and jump-6 boats restricted to trunk lines. I can see absolutely no good reason to eshew jump-6 altogether. (Well, except for the reason I've suggested before: When jump-5 was invented, the Navy glommed onto it and kept it for itself for as long as they could. By the time it was released fro civilian use, the Imperial Bureaucracy had been relying on NavyNet instead of the X-web for a generation and there was no reason to upgrade. Same thing happened when jump-6 was invented).
Or why if using Book 5 it's not 100tons with a lot more room for high maneuver, cargo, weapons, crew, etc. by utilizing drop tanks?
Drop tanks (or rather, the special capacitors you need to use them for jump)
are a genuine new invention.
No, I think it makes little sense from a design view, but I don't think that's the reason the ship was built the way it was.
The in-universe design view IS the reason the ship was built the way it was... in-universe, that is.
My point is redesigning it to include a maneuver drive or whatever "because you can" breaks the background and I don't think one should if still calling it the OTU.
True, but the breaking was done when the rules were changed to make it inexpressively stupid to design it the other way, precisely because you can
and always could design it with a maneuver drive.
Making it J5 doesn't really break the background while allowing a legitimate use for all that "extra" tonnage. It's a capability that exists but isn't much used nor really changes things much even if it were.
But the disconnect comes when you ask "Why is the capability not used?" There's no good reason why not.
Book 5 1st edition had bis models all the way through. So you could, and it would be TL11. And (if I recall correctly) Book 2 1st edition and Book 5 1st edition didn't require that computer model equal Jump number, you just needed enough computer to run the programs, so you could get by with a model/3 TL9 for J4. Not sure why a model/4 was specified originally, unless it hinted at design rule changes to come or because there was probably an extra ton of space, so why not.
The way I look at it, there's a "real" way that shipbuilding works in the OTU. Every design system we've seen has tried to emulated that one single "real" way, though some of them were egregiously wrong in various ways (HG has ridiculous power plant fuel consumption, MT has the jump fuel requirements wrong, TNE uses fusion drives instead of thrusters, GT has that 20% for streamlining foolishness, etc.) I don't want to shop around and find the design system that works best for one particular ship and then turn around and use another one for another ship. I want a unified system that's closest to "reality" without being overly complicated (Is the lower limit for a jump field really 100.0000 dT or could you actually make a 97.8765 dT hull in "reality"?

). Though sometimes it's not easy to figure out what that "reality". Do you really need a factor 5 computer to compute a jump-5? What IS a factor 5 computer?
Not that it's a bad idea, actually it would be a good idea, and the artwork supports a maneuver drive (but art and fact rarely match in games and novels). But again, it would be too big a change. No longer is the poor Scout stuck waiting in his X-Boat for the Tender. No more dramatic plot potentials in the dilemma. I'm all but sure that was the reason for the setup so imo that's how it should be.
Too big a change in what sense? Would it invalidate any official adventure or amber zone? I can't recall any that involved a poor Scout waiting in his X-boat for the tender. Would it invalidate the X-boat system? I can't see how. Just what would have to be retconned (other than the description of the X-boat itself) to make such a change work?
I'm not saying that I don't regret the necessity of getting rid of the drive-less X-boat, but not nearly as much as I regret leaving it be without an explanation that works.
Tenders are another bit of a design mess imo. But I've got to go so I can't get into that here right now. Not even sure I want to
I've never studied the tenders in any detail, so I can't comment on that. But I'd be happy to read more about it.
Hans