One reason to use tubing is that you can slide tube into tube for adjustable mounts. We use 1 3/4 .120 wall tube as you can slide 1 1/2 inch tubing into it. For some of our mounts we use 3/4 scedule 40 pipe instead of one inch tubing as it is .50 of an inch larger and as such a bit stronger. We have also found that even with the proper dies for the bender the 3/4 inch pipe bends nicer then one inch tubing.
One reason I can see that some one might want to use pipe rather then tubing is that inexpensive benders such as those sold by Harbor freight come with pipe dies and not tubing dies. As long as you design the frame for the tubing or pipe you are using I see no problem running pipe rather then tubing but you must design for the material you are using. For 4 X 4 and such one reason there may be debate is regulations for entering events may state what roll bars and the like must be made from a specific tubing. I know that when I use to build rally cars the SCCA required us to use specific tubing sizes to pass tecinical inspection on the roll cages.