There are some articles written by Tomiki to be found at JAA(USA) site and Shodokan Honbu both of which can be linked from my Dojo home page (see below).
He outlines his reasons pretty clearly but one of his points (excuse the pun) is that by placing your training (this includes both Embu and Randori (tanto and toshu)) in the context of a greater whole you avoid fragmentation. It becomes clear where the quality lies. I am not sure how successful that's been since we too have our fragments but I do believe the competions have helped and over the past few years several splinters have returned to the fold.
I know that only indirectly answers your point but yes competion provides a basis of quality control. Remember it's not just randori.
Just to expamnd slightly on why points. In the old days shiai basically meant to the death. This is not a training option for me thank you. Even during the days surrounding the formation of the Kodokan there were matches between Jujitsu schools and the practice of just going out onto the street to pick fights with the biggest bruiser. Both of these latter options were brutal but generated superbly trained individuals. In the absence of staying alive or in the latter case last man standing contests - points offer a reasonably safe alternative. A short term goal so to speak. Putting something on the table, life, your arm, moving up the rung, adds something to the equation that would not normally be there. Points is infinately more safer than life.