The attack on assits is used like this.

If I have it switched off and I use a macro the is /assist Mytank the character running the macro will target Mytank's target. If it is switched on the charactcer will target Mytank's target and start attacking, Melee and Hunters will use what ever gun/bow/xbow/etc they have if at range from the target, and casters will start wanding. If in Melee range melees and hunters use weapons and casters use staff/mace/whatever...

The other thing is I think not something you can stop, if your target comes into melee range on non-casters the non-caster will always switch to a melee weapon, casters will carry on wanding. In the case of shamans, because IIRC they can't use wands they'll switch to a melee weapon even if you don't want them too, unless you have the attack on assist switched on...

I think. : )