2 umpire mechanics, going out on a fly ball

[Dakota]

In 2-umpire mechanics, as the BU, under what situations (your position, runners on base, flight of the ball, etc.) do you (a) always go out; (b) never go out, and for those situations that fit neither, (a) nor (b), how do you decide which to do, go out or stay in?

In the 2-man system with runners on base, it seems to me that you give up a lot (2 umpires covering tag ups, calls at the bases, etc.) for a marginal benefit (since we're only dealing with 200-225 feet to the fence), so it should be done when there is enough benefit to give up the second umpire.

What prompts this is I recently had a partner who seemed to turn his back and make the fly ball call on nearly all fly balls, even pop ups into the shallow outfield.