I suspect that this is the reason the Borg initially made the TW hubs/network.
Once you make it, you don't have to spend power on a starship to travel at TW - so its more energy efficient in the long run.
Though, there ARE benefits to being able making your own TW conduit on a starship such as personalized course corrections, and going where you want to go.
But this is probably where most travelled areas will have pre-established TW conduits, while the individual ones would be generated by ships to get to unknown or different locations (its still a massive time and energy saver).
For civilian travel w/o needing a complicated engineering / science degree, establishing TW Conduits as "InterStellar Freeways" for civilian travel has a HUGE application for trade. Imagine getting to your destination w/o having to spend a giant boat-load of your energy, the galactic trade implications alone are HUGE.
On the other hand, QS v2 is similarly fast (if not A LOT faster) than the regular Borg TW network... and it can run off pre-existing power source with very high efficiency... only downside is use of Benemite crystals which decay (which I'm sure can be bypassed with some R&D - and for less lengthier trips, you can just use V1 of QS which doesn't need Benemite crystals to start - just reinforce the SIF to compensate for the Quantum stresses - or limit yourself to about an hour inside the Slipstream v1 - which would still allow one to traverse 25 Ly's in that small time frame).
Even if it takes 2-3 years to grow a Benamite Crystal, we have farms that mass grow things in bulk, we can grow Benamite Crystals in bulk over time like people make wine for fermentation. It's really a non-issue. And decaying, all that does is bring more value to existing Benamite Crystals and until you can freeze the Benamite Crystal's decay some-how and it only decays when in use, then it becomes a very valuable / rare commodity for Transwarp Travel.
It's hardly the end of the world and I can already come up with numerous ways that growing Benamite would become it's own industry.
QSS.v1 would be the more common use-case since anybody can use it and it's fast enough. Just hope your SIF generators are up to snuff or you have to exit prematurely to let your SIF emitters recharge before you hop back in. That's a small down-time issue and great for in-universe / literary balance. It creates a time-limit for usage of QSSv1.
Having limits isn't a bad thing IMO and creates interesting universes. Just like SporeDrive is useless w/o the Biological Navigator. It's computer-aided jumping distance is only good for local STL tactical space combat.
The 'Underspace' is there... but the problem is that detecting and using it are a different ball game.
Not all species know of their existence and it seems that they do NOT extend to the AQ, or to SOL (in 'Dragon's Teeth, the Vaud'Waar mentioned that VOY could find itself 1000 LY's closer to home... which wasn't a lot on a galactic scale when you think about it - so the underspace network seemingly extends throughout half or two thirds of the DQ - and there was no mention they go to other quadrants).
Nifty and handy, but not terribly useful.
We don't know how far it extends, it's largely unexplored and most of the captain's kept the knowledge of it's route hidden in their own brains. Not much of a map if you aren't sharing the routes.
For all we know, it could extend across the entire milky way, nobody has bothered to fully map it.
The Spore Network seems to cover the entire Milkyway within the Galactic Barrier, there is no reason why UnderSpace might not do the same.
Its possible that the Jurati Borg may have helped the UFP create some TW conduits for well travelled areas of the AQ. As we know, some TW conduits already exist in the AQ, but those were likely from the 'Descent' Borg back in TNG - and they seemed relatively slow in terms of speed - possibly due to lack of maintenance.
Who knows, but TW conduits within the TW network seem to be held open by InterSpatial Manifolds. Otherwise why else would Janeway need to destroy them to collapse the TW tunnels/conduits.
If left alone, I believe that TW Tunnels would naturally close over a span of hours, depending on how big each tunnel is, the larger the tunnel, the longer it would take to close, obviously the smaller the tunnel, the faster it would close. So it doesn't close at a linear rate, but one that accelerates as the tunnel gets smaller. That would make sense logically and explain why the Borg insisted on placing InterSpatial Manifolds across the TW Conduit Network.
If it's holding the Tunnel open, then that explains alot about it's use case.
