The issue isn't stress from towing people on tubes. Any ND or newer MC tower is plenty strong enough.
The problem arises if the tube tanks under water and turns into a parachute. I've seen someone snap a heavy tube rope when this happens.
As long as you're watching what you're doing its ok
'06 X2 MCX
"I understand why some people may not want to do this the way I have recommended but I can't understand the death grip some people have on a toilet plunger with a hose fitting." -JimN