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In Fallout 1&2, ending your turn with unspent AP gave you a bonus to AC (defence).
In some other games, contingent actions can be set up, e.g. pointing one's firearm in a particular direction or arc, ready to shoot if an enemy enters that area. Paying AP for a potential action.
In my homebrew tabletop, Sagatafl, AP are also paid to perform defensive Re-Actions such as Parrying or Dodging.
It might also be worth examining another thing, the relationship between AP and movement. Fallout 1&2, for instance, sticks very close to 1 AP = 1 of movement, so that approach is incapable of simulating encumbered characters (except in an extremely coarse-grained way that if you're severely encumbered then it's 2 AP per hex).
If you want to expand upon the possible usages of action points in TBC, feel free to add a new section. That's what the Analysis/ namespace is for, after all. :-)
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