Avenging Spirit is a paragon fighter feat that provides an action point whenever an ally drops:
Benefit: When an ally of your level or higher is reduced to 0 hit points or fewer, you gain an action point that you must use before the end of your next turn or the action point is lost. You must see or hear the ally when he or she is reduced to 0 hit points or fewer to gain this feat’s benefit.
How often can a player with this feat take advantage of the action point gained in this way?
Looking at this suggests no more than 1/encounter. Which suggests a secondary question: how can a fighter take best advantage of the extra action points gained in this way?