With Willson he's got 40-50 games he'd be at catcher. Then with injuries else where, let's say PCA got injured, Cody slides to CF, Seiya to RF and Willson into DH. If you bring in a DH/OF there is no 40-50 game buffer like there would be for catcher. You'd be solely dependent on injuries/rest to get them ABs in that scenario and most position players rest what, maybe 10 games a year compared to 40/50.