I just received a response from Paul Butler of Dorna Sports to my inquiry on gridding. This is his reply:

Dear Mr. Koblichke,
I have been asked to reply to your enquiry to Dorna about the evolution and merits of various starting grid solutions. As you know, since the road racing world championships began, the starts have evolved from push starts to clutch starts. In the early days of push starts the rows of some 6 riders were parallel - the push start delay and much less power allowing for a graduated departure. Since the introduction of clutch starts and much higher power outputs and acceleration, grids have evolved into "echelon" format. This evolution has been based on safety considerations - no rider is directly behind another on consecutive rows, and on reward for practice achievement - the faster riders are ahead of the slower ones on each row of the grid.

The International governing body, the FIM, has a publication "FIM Standards for Road Racing Circuits (SRRC) which illustrates the dimentions and layout of starting grids for homologated circuits. I hope this is of help to you. Paul Butler.

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Given his explanation and after looking over documents from the FIM, I see no real argument for or against parallel or diagonal gridding other than diagonal rewards riders for their practice / qualifying times. Staggered gridding (echelon style), which we do now, is all about safety

http://www.fim-live.com/en/fim/fim-official-documents/