Except the routes are all to populated systems (never seen one on a map that wasn't in a system with a mainworld). Why tenders and not transfer stations? (Answer: Tenders make sense for establishing an ad-hoc link until you decide to build a transfer station there, even in deep space - a breakdown design for the transfer station would let you build one even in deep space... but no official X-boat routes I've seen do anything with that.)
That's a nitpick. I just don't see using a ship when you could use a station.
Also, you could probably get a big rock cheap in a lot of places, so some might well use planetoids/asteroids for economy.