think about Tony Parker and Nate Robinson..both of them can drive the ball well but only one of them can finish very hard lay-ups..driving gives the player better opportunities for a shot, which means better percentage but it is nothing without IS and JS skills..
IS matters for the guards but so does ID..you don't have to train it because there are not many guards with good ID skills..if you play "look inside" or "low post", your guards will get inside many times..zone defense does not change anything, still PG defends PG, SG defends SG..
don't you watch your games man..?3-2 gives a boost to your outside defense rating and 2-3 vice versa..but if you watch the game you can see that your players are still playing man-to man..have you ever seen a center blocking a guard..?
If inside shot is important for that key ability, then guard trainers should be able to train this skill. Being a post player and being able to make a layup are very different. Same for being able to defend a layup vs defending a hook shot.