Space mines (and even their own entry in TacOps says outright) are known to be of questionable usefulness, so unless you have literally no other means of defending yourself people do not bother with them. Deploying space mines requires a space mine dispensor, not a tug in any case.
Having them take up a collar in a combat force is of debatable usefulness. On the one hand, they offer some cargo space and other advantages with better weapons and armour than most DropShips. On the other hand, if you want cargo why not just carry a
Mammoth or
Mule, and if you want an Assault DropShip, why not just carry one? WarShips are already precious enough assets that compromising their ability to undertake their duties is not going to hold much appeal.
Post Star League/Age of War tugs are mostly seen at naval yards or assigned to salvage units. The
Star Lord was fluffed as being equipped with magnetic harpoon thingies to assist in recovery of DropShips, so nations that follow the practice of its TR entry with reserving some to conduct salvage operations would probably assign some tugs to them permanantly as well.
So far as grappling an enemy vessel under combat conditions, the rules for them state that if the target resists the grapple (ie, spends any thrust points when the tug attempts to mate) the attempt automatically fails. There's also the problem of how low Tug thrusts can become once they have glommed on; if a 6/9, 15kt
Octopus latches onto a
Scout at 90kt it can only move them at 5/8*. A
Fredesa drops it to about 3/5. A
Vincent reduces it to 1.25/2.25. Once you start looking at "proper" WarShips you are looking at spending multiple turns just to counter their current velocity, never mind redirect their heading.
*Technically 5/8 and a bit, but the .25 thrust is negligible for the example. Other examples rounded similarly.
When the Werewolf was escorting Peter's coalition into Tharkad, the Warden Wolves had a Elephant tug as one of its dropships, so there might be some doctrine for it.
OTOH, the Wolves were meant to be scrambling for equipment. I'm not sure having a relict of the Star League around some centuries after it was constructed demonstrates too much, especially if the Wolves were going in to gather any and all
isorla on offer.