If they are double braced like shown, I don't think they will have much a chance of moving. This is especially so once you wrap the frame in siding, SIPs, or what have you.
However, my thought process is thoroughly influenced by the German methods, which often times involve no pegging or pinning at all but rely on loading forces to hold things together.
I would tend to say that it would in fact be good to do as you say and eliminate the wall plates and instead brace to the top plate. I can think of a number of reasons why this would be a better setup than that pictured.
http://www.tfguild.org/joinery/part1.pdffig 1. at that link shows the setup that I think of when thinking of a frame such as this. fairly straightforward, and proven.
It makes tremendous more sense to me to brace to a primary structural member such as the plate, rather than a redundant, secondary member like the wall plates.
