A low tow has several uses.
1. It is easier to fly and hence less tiring for the glider pilot so useful on long level retrieves.
2. The Australians use it in preference to the normal tow (above the slipstream).
3. If the tug is towing two gliders at the same time then one above and one below is appropriate.
4. When training glider pilots to aerotow so they know how it feels flying up and down through the slipstream if they get out of position inadvertently (uncomfortable!)
Disadvantages:
Tug pilot has difficulty seeing a glider in low tow in mirror.
If glider releases in low tow then the rings on the end of the rope may smash the glider's canopy so the glider must climb above slipstream to normal tow to release.
You are assuming the load from the rope to the tug is much more than it really is. For example a 500 kg glider at 70 kts with a glide angle of 40:1 has relatively low drag. The glide angle is usually known at lift divided by drag so in this case the drag is 500/40 = 12.5 kg in level flight. If it is climbing then one has to add the load for the effort to pull a body up an inclined plane - not something for here.
If the glider is in normal tow above the slipstream, with the tug in trim, and the glider moves down to low tow then as it does so the combination will tend to speed up slightly (less pull on the rope) and then stabilise when the glider stops descending. The stick load difference in the tug, bearing in mind that there is about 50 metres of rope to the glider, is negligible and there is no need to retrim.