Yes, I am aware people do use such missions for training (I swear I even saw someone say so on this very forum...)
Not particular fan of "obsolete missions" either. Having said that, I'm one of the guys who use low level missions to train rookie troops (whether recent hires or brand new claim pros). If I know the mission won't be too dangerous, I usually sent a mixed team: 1 or 2 high stat veterans, 2 medium stats, and the rest rookies, or someone who was idle for too long.
This works quite well. Case in point for zombies. Once a defensive perimeter is set, everybody reaction fires until the battle is won. Same for bio entities (strange animals), except when the MiB shows up with their vessel, large crew and heavy weapons, then it's "ouch time", and my veterans hopefully earn their pay.
In fact, I even hide "Exalt Operations" so I can get the occasional "Safe house" and use mostly rookies for training.
Yeah, I have no idea where people get this idea from. It makes zero sense.
I don't have idea either. Yet somehow I have the same feeling. Like I get X amount of missions for the month, and they are spread among multiple possible scenarios. The more scenarios the less chance the ones favorable to advance the plot to appear.