I tend to think it is more the WHIP.
Recently, I've let HAL run my bullpen, generally $750 guys, for a good part of the season and then started to depend on the guys it was using.
Looking back at one season where HAL kept going to Kipper early and not Dawley, I see that Dawley's ERA is a full run better than Kipper but Kipper has the better WHIP.
I'd watch it on my upcoming season, but I have broken with tradition and gone for a big stopper, Henke, and don't want to waste him.
However, anyone going with a cheap bullpen might want to let HAL manage it for the first part of the season, save the stats at that point. Then at the end of the season see how which card the player was impacted on his usage up to the end of the test period.
Should answer the question once and for all.
On the original point, about pinch-hitting, I see no rhyme or reason for HAL's decision. I dumped Van Slyke after four games because in two of them, HAL pinch-hit for him against righties.