Since the radius of "square" corners is a constant, longer lines make the pattern look better and score better. So you need to select equipment that lets you use the full 70 feet. Anything less is suboptimizing.
This is probably why the trend has been toward larger planes.
Having said that, though, for any one given plane it would make sense to fly that plane at it's optimal line length. We could then argue about whether it's a good idea to go to a bigger motor to allow longer lines, and at what point that no longer makes sense (i.e., no, I'm not going to fly a Ringmaster on 70 lines with a 46LA. Just -- no).