The advantage of using more "tubes" is that the track then requires less supporting, because the extra "tubes" act as a brace. Intamin decides which would be safer and more cost effective, more tubes or more supports, of course each situation differs.
Rita: QoS" is 3 tube all the way around the track and that has Vertical Gs of up to 4.7 which is pretty forceful.
But did you notice the support bracing?
Whereas coasters like Millennium Force have places where 4 tube track is not necessarily needed like on this airtime hill:
Yes and no, they decided they didn't want as many supports, so that it would look cooler and more clean cut.
The track type is based on forces and support options, NOT speed, a train traveling on a straight away track at 200mph isn't going to need anymore bracing or "tubes" than a train traveling at 100mph on the same track.