That would be my guess too. Smith may not have had many opportunities, but he wasn't pushing the pile in his few. Now with Rene seeing the field (and looked pretty quick), looks like Smith is down to 4th string. Barring injuries (knock on wood), wouldn't plan on seeing him the rest of the year.
I am not so sure of Smith being passed by AAM yet, but it could very well be the case. Smith has seen the field a few times this year and wanted to give AAM a look too. Now that he has game film and has seen them both in real-game situations, he can figure out who is better. JMO.
It could also have been situational? Btw, I was somewhat impressed by AAM Saturday. I'm not sure about his conditioning based on the "look test". Then again, if you looked at Craig Heyward, you probably wouldn't have come away impressed either (not comparing the two, mind you).
I'd guess that the intention was to get some game time for as many RBs (and WRs?) as possible to provide film for evaluation during the bye. But that went out the window along with our big lead and they were forced to lean on the starters longer than they wanted to preserve the victory.