I'd tend to agree with this assessment. My guess is that a samurai would have his attendants normally carry such things, and put the sashimono on when action is immiment, or for some reason when identification was important. I am not convinced that samurai wore the mon of their daimyo all the time, they may have been more personal, or for lower ranked samurai they may have been the mon of their direct master, rather than that man's master. A look at the known Takeda army mon variety shows a lot more variety than the four diamonds type.
I think its less likely than is generally depicted on wargames tables that ashigaru wore sashimono. I imagine that was limited to kashira (sergeants/leaders) more for the easy identification of them by soldiers and higher ranked leaders. Ashigaru often had mon painted on their body armour or helmets, especially if issued from their lord's armoury (or indeed on little streamer flags suspended from the arm, though this may have been more of a samurai practice, I am not sure).
All of this is just my impressions, I can't say I've looked into it in great detail.