I can't really think of what it would add except more coverage on the beams. If I'm doing Small object avoidance, I want to look in front of me, more than on the sides.
Consider that it was designed to detect submarines and not for small object avoidance.
Maybe I was thinking it would work similar to what heuhen posted about the SQQ-90 with one high freq sonar for mine detection and 1 mid freq for sub hunting.
So Ideally, in a dual band sonar system, you'd have one high frequency for SOA and one mid frequency for Sub hunting. If the two planar arrays share the same frequency bands as the cylindrical 53C, that's even worse.