They barely play more than a series in the 1st place, if at all.
I seem to recall Keenum and Benny out there for more than one series.
I hope you aren't confusing our first string offense/defense with "players guaranteed a roster spot" because there are plenty of players that are not first stringers who are guaranteed a spot. Goff would fall under that category. We can make the argument for Mannion as well.
And I'm quite sure they play more than one series in preseason.
I certainly understand that late round picks and UDFA's are a focal point of preseason but you can't say that it isn't also a chance for our rookies and new players to get a feel for our playbook and speed of the game.