Unfortunately HAL (the game AI) doesn't manage very well when it comes to platoons and pinch hitting.
It doesn't do well with no-DH teams and double switches either, in fact I've never had HAL pull off a proper double switch when I have a lead off platoon and the pitcher is pinch hit for with the other half of the platoon. He never seems to get the pitcher into the leadoff spot and leave the other half of the platoon in the 9 hole.
If you are behind he will pinch hit, but with the wrong guy sometimes (a different batter than the platoon mate)--if you are ahead, HAL will almost never pinch hit.
MY biggest gripe about the online AI is it seems to try to come from ahead to lose when you have a lead--it never pinch hits in high leverage situations when you have a 2 or more run lead and want more runs.
In fact I had this situation come up tonight on a new team with the 2014 card set---My team had let a 9-2 lead deteriorate into a 9-8 lead--bottom of the 7th I have the bases loaded and .79 SS 2-e18 Romine (3L) is at bat--I need more runs now--the other team brings in a RH pitcher to face him--I have the other half of the platoon available .91 SS e-19 Segura (4R)---better yet I have Delmon Young on the bench a .300 hitter, .450 slugger--Does Hal pinch hit??---NOOOOOO-- he lets Romine hit.
http://onlinegames.strat-o-matic.com/game/426421/143wait--- wait a second Romine hits a Grand Slam on a 3 9 roll (Young would have struck out)---HAIL HAL---HAL knows all.