Nadir-SAR would look very confusing, full of ambiguities, and probably unexploitable since the ranging part of RADAR would not be able to distinguish the left returns from the right returns -- that's why SAR is side-looking.
even with a strictly side-looking geometry, images collected closer to nadir (steep/low incidence) start exhibiting some artifacts such as nadir returns [1].
I've never seen "close to nadir" radar images... wouldn't the ground fold over itself?