If all the electricians you know are saying an out-building with a sub-panel does not require a ground rod you need to find new electricians.
A detached structure most certainly DOES require some form of grounding electrode. This is regardless if it is fed by a 3-wire or 4-wire feeder.
I think what the guys you know are saying is that you should use a 4-wire feeder which has a grounding conductor run with the feeder conductors. This is not a ground loop though.
Most times a 4-wire feeder is required by code.
A ground loop is something you DO NOT want. The point is that if there is a compromise on the neutral conductor the ground does not carry any load current.
Sometimes I feel like if I answer any more questions it is like someone trying to climb over a fence to jump off a bridge and me giving them a boost.