I don know if its usseless, AG also affect other factors than AP regen (bleed resistance, HR, AC, and others)
But, Im not a big fan of traits that affect a specific SPECIAL for bonus or drawback, because they actually turn into a generic SPECIAL bonus. I would rather like to have one or two traits that gives a generic SPECIAL bonus/drawback (like a balanced version of gifted). And replace those SPECIAL modifiers for the traits with other effects.
Looking at this particular trait, I think that a small frame person not necessarily have to be more agile, but due to their small size they may be harder to hit. An AC bonus can make sense. But now that AC its being revised, another option may be to have the max chance to hit reduced by 5% against you. So regular players max chance to hit you its 90% and 85% for chars with fast shoot. The current drawback fit the theme of the trait