the downside of a "station with Tug" is tactical. specifically, while the tug gives the station some mobility within the theatre of the solar system it is defending, in battle the tug doesn't offer much maneuverability, and it leave a rather large hole in the station's defenses if it stays docked (the tug blocks the weapons fire, IIRC)
so while the tug would give a defensive station the ability to be moved around a solar system, in battle the station might as well be stationary. and ultimately the ability to be relocated is of little strategic use, since there are relatively few places worth defending in most solar systems (planets mainly), and even Dropships and full warships cannot move fast enough to reach an enemy jumpship jumping in or a enemy force burning towards a world if they are not already pretty much right on top of the spot they need to defend. the IR signatures of an incoming jumpship last at best minutes, and it would take days to get across the system (weeks for a station+tug set up, especialyl if the station is large enough to be useful.), and even at high G's you'd need the better part of a day's warning that a enemy force is burning towards a planet if you want to even arrive at the same time. and sustained high G's are not really an option with Tugs (too much stress on the linkup), and the crews of regular dropships and warships would not be very combat effective after several days at 2+ g's.
so while it is an interesting idea in general concept, it doesn't really work out as viable with the technology available in the battletech universe.