Humans in the Android Arc are strong enough to be considered a mini-aid in their fight against 2 more powerful enemies than Future Trunks SSJ (first appearance).
I have:
Krillin (Androids / CG) = 16.384% Freezer (Namek)
Tenshinhan (Androids / CG) = 16% Freezer (Namek)
Yamcha (Androids / CG / Boo / DBS) = 12.5% Freezer (Namek)
Chaoz (Androids / CG) = 1.6% Freezer (Namek)
I think Yamcha reaches his maximum power in the Android Arc, then as CC said, he trains a little just to maintain his power. As for Krilin, Tenshinhan and Chaoz, they are still training. Personally I believe that Krillin trained arduously after the GC and before joining # 18, so impressed # 18.
Krillin (Boo / DBS) = 44% Freezer (Namek)
Tenshinhan (Boo / DBS) = 40.96% Freezer (Namek)
Chaoz (Boo / DBS) = 2.56% Freezer (Namek)
I think that Krilin and Tenshinhan went to the fight against Freezer in FnF, because they felt useful in that combat. Yamcha knew that something useful would be in that fight, but when thinking that its little help would not be very necessary, because they were Gohan and Piccolo. Yamcha preferred not to risk his life unnecessarily.