I'm also in the camp of appearances, but I'm not sure if it's purely games-played or whether it's innings volume. Because the game does track total innings for position players....
If you fill your spring training roster 70/70 with let's say 30 pitchers and 40 hitters (8 positions x5 guys at each) and then (auto-)fill your hierarchies for pinch hitters, defensive replacements, and player rest, and then set your manager settings to pinch hit-5, pinch run-5, double-switch-5; then set your DEFENSIVE SUBS AND REST: defensive subs when up by 1 in the 7th (or 8th or whatever), and set Use Rest when winning by 1 in the 4th (or 5th) and when losing by 1 in the 4th (or 5th), and then go to Player Settings-> Pitchers and set everyone's pitch counts to whatever is the minimum and set everyone's pull rating to 5
This is obviously extreme and ridiculous and who has time for all that, but if you did all of these things you could conceivably use all 70 players in every single game.