Well because usually it's a matter of time before things go back to normal for those type of D's. They usually go on "runs"
If you can insert your #1 and 3 D back in the lineup, you do it no matter what!
A long time ago I learned that in any sport, when a key player goes down and a backup comes in several things happen....
A. coaches often simplify schemes and find ways to protect and minimize the impact the sub has to make...
B. Players tend to play more resonsible games around newer players... they take fewer risks and liberties..
C. The players themselves tend to take one of two paths, they either try to do too much to prove their worth, which often ends badly, or they play a simple game within their means and look fine...
D. It almost always puts a level of uncertainty in other teams gameplans for a bit....
After a while teams get used to the sub and start expecting more from them... then the above statements start being void... he tries to do more, teams start countering his strengths and focusing on his weaknesses, players around him start taking more liberties cause they trust him...
Then either he proves he belongs as a starter or he faulters and his flaws are more prevalent.
Guys like Ruh or, I suspect, DeSmith, are probably not everyday players. The look good for a couple games or maybe even a month.. but eventually they are asked to play with fewer safeguards in place and they look very average or worse sometimes...