concur. I think the best answer is to have a bullpen full of good relievers, so that you don't have to worry so much about which one to use when.
Since that's difficult, to say the least, you're left with how to use what you have. And planning for use based on situation is just extremely difficult to pull off.
Yeah, it's hard to predict what will happen in a few innings' time but there are a few things a manager can do to create a more flexible bullpen.
For example, it's the sixth inning of a tie game. The bases are loaded with no outs.
Call in your best reliever. Just do it. And teach your relievers that those situations will occasionally arise. They're the "eighth inning guy" 90% of the time but when an obviously critical situation arises earlier in the game, they should be ready to get the team out of the jam.
Far too often we've seen that kind of sixth inning situation pitched by the "sixth inning guy", which makes zero sense. Deal with the later disaster situations when they arise, if they arise at all. Use your best guys earlier in the game when it makes sense.
The manager doesn't need to do that often but there's some wiggle room in there I'd like to see explored more often; the "obviously critical" points of a game we all see unfold once every couple of weeks.