As ever a useful analysis of the published literature on Practical Fragments, "Evaluation of 3-Dimensionality in Approved and Experimental Drug Space" DOI.

The true need for topological diversity in feedstocks and final drug molecules remains unclear given the overwhelming number of linear and planar drugs. The question remains as to whether more 3D compounds represent attractive and untapped therapeutic space, or if more linear/planar molecules are indeed the best topologies for bioactive molecules.

I came to a similar conclusion when looking at published fragment hits.