It would be cool (especially with STF characters) to test your damage and retrieve some actual damage value back. Have them located in a somewhat more isolated location than where duels usually take place.
~3/4 for naked/light armor, medium armor, and heavy armor/plate. They have unlimited health and stay in place, but when you hit them you get a message concerning how much damage you dealt (maybe even speed bonus).
It could help people with timing of attacks, target practice for ranged, or give info on the effectiveness of your STF character with actual figures.
I doubt a few NPCs with this specific purpose will be taxing on the server or anything, and duel doesn't usually have more than 20 people unless it's right before a strat battle.