I've always been told that the reason most helicopters are flown from the right seat is that helicopter's are inherently unstable in pitch and roll. so flying from the right allows you to keep hold of the floppy cyclic while you release the collective momentarily to carry out other duties, ie radios etc. Flying from the left seat means there is a need to release the left hand from the collective to cover the cyclic, whilst you use the right to operate radio's etc. tricky especially if you are right handed.
This is a very simplistic argument, as for the weight and balance argument, clockwise / anticlockwise that may be an issue for some small helicopters, but take the army lynx and Gazelle as an example, one has a clockwise rotor, one has an anticlockwise rotor, Both are flown from the right seat.