As a Paramedic, myself, and former volunteer EMT I say that if the ambulance is on scene I am not needed for help in that way, but I may be needed to help with family members. If it is something outside my neighborhood I keep going, if it is someone I know I go to one of the MFR's that isn't busy and ask if the family needs anything. Sometimes they need a babysitter, a ride to the hospital for themselves when upset, sometimes a good hug, and sometimes nothing. If there is an entrance to the house that won't put me physically in the way, I use it.
On the other hand, if the fire dept. is not really trained medically I may stop to see if they need help, though honestly, what am I going to do? If airway is being managed then there is nothing really to do. We should not give the patient any of his meds, unless aspirin or 1 more nitro and even the nitro is iffy without a blood pressure, we can't start an IV, intubate, etc. just as long as the airway is being managed.
To be brutally honest, and remember, I'm finishing my nursing program so I'm not slamming nurses, about 3/4 of the calls I've been on where a nurse is on scene have been nightmares. Some have been fantastic and I've been really greatful, but....I have seen some of the most boneheaded "treatment" you can imagine. Again, if the family needs help or fire needs help, I'm there. Once an ambulance is there I back off and let those who are specifically trained for pre-hospital work do their thing.