Good point and maybe it carries the day.
But if Wallner were added to the roster, at the expense of Farmer, then against a righty you start the above group, and when a LHP is brought in to face Larnach you pinch hit the righty Margot, and then next time around the batting order you have Wallner to counteract the RHP relievers the rest of the way. (Or start Wallner and have Larnach as sub, they're kind of interchangeable even if not identical.)
Don't know if it stretches the talent too thin in the infield. But Farmer just isn't getting the job done at bat and he's no longer better than average at any defensive position to make up for it. The subtraction of Farmer would leave a 40-man spot open to bring someone up (or in) if the need arises.