I have been on both sides of this type of situation. When pulling a person on a towable or on skiis, a spotter is required, however, the boat operator still has his/her work cut out for them. They WILL BE distracted by not only the course ahead, but the couse of his towed human projectiles, the spotter's reactions, waves (not the Hobie kind

), and a number of other factors a non-towing powerboat operator need not consider, or at least to the same degree. And an evasive manouever by the pulling boat is not neccesarilly going to be followed by the object in tow. Rope swing, momemtum, etc, play a big factor.
I have pulled waterskiers, tubers, and let me tell you, it can be quite nerveracking. Especially if there other powerboats. Now, the non-powered craft are pretty to avoid. I alter my course slowly and simply stay WAY clear of kayaks, canoes, sailboats, and the like.
While sailing I typically do not end up bieng in the way, as the powerboats I encounter seem to steer way clear of me, I try to keep my actions predictable. There have been a couple of times I changed my couse on my Wave as a courtesy, to allow a the boat w/ skiier in tow to stay on their apparent course and destination.
The lake on which I do most of my boating is fairly small, and the other boaters seem pretty aware of each other, and simply stay well away from each other. Of course, this may not be the case on more crowded water bodies, so as many others here have pointed out, never assume another craft is going to follow ROTR, especially if they are towing. (Again, read DISTRACTED).
Personally, I agree that the powerboater was irresponble, pointing his boat w/ tube in tow into the path of your slower sailboat. It was not like he could not predict where you were headed. I think you were in the right, but always remember that a fair amount of boaters out there are cluless re the ROTR. Esp the PWC kind

. Don't get me started on that one
Just my $0.02