I went through the different classes and totalled up what the average repair costs of all the components would be if you did in-the-field repairs (at $5 per pt, $50 per wheel), if they were completely wrecked (total damage, and all wheels broken), removing the top and bottom results from each class component average (so that the likes of the Tatarin doesn't sway the Scout class too much, etc)
This is just from the base-game trucks, no DLC ones.
I came to the following average :
Heavy Duty: 3285
Compared to the current Hard Mode in-garage "full repair" set costs:
Scout: 1500 (60%)
Highway: 1500 (48%)
Heavy Duty: 2500 (76%)
Offroad: 2500 (69%)
Heavy: 5000 (112%)
The percentages are the percentage amount you need to wreck a vehicle before it's break-even to tow it to a garage and do a full repair instead of in-the-field.
So, Highway is the only class where it seems remotely likely to be worth doing.
...while Heavy class trucks in almost all cases can never get to a state where they'll be worth repairing in the garage.
Might an adjustment be in order?
I think making the in-garage costs maybe 50% of the class average total-wreck cost might just make this a useful function - if a truck is damaged beyond a certain point, it may then be worth your while towing it back to a garage instead of repairing it in the field. (That would be, rounded to the nearest 50 for each class: 1250, 1550, 1650, 1800, 2250)
...or, the in-garage costs could just be changed to match the repair-point costs...
Edit: Scratch that, changed to be proportional, but 20-40% higher than repair-point costs, otherwise we'd be back to service trailers being largely pointless again.
Edit2: Actually, giving it more thought, I think that better-balanced, this system could give an interesting extra layer - it could actually be advantageous financially to endure a bunch of suspension and gearbox damage, and even a little engine damage, to save money taking a truck in for a "full-service", which would encourage a more "push-your-luck" approach.
You could selectively repair engine damage to avoid increased fuel consumption, and leave the rest.