Let me see if I can answer your question as a former experienced police officer.
You're speaking in absolutes and you can't do that. Generally speaking if I am in a potentially and I emphasize the word potentially violent situation yes I would prefer to be the only one there with a gun... It's a matter of control and that's primarily what you are taught to do, control the situation. However and this is a very big however, if my life is in immediate Danger and I am losing or it appears as though I may lose I would have to be an idiot not to want a legally armed citizen to assist me.
The fact of the matter is I can't name an instance where a legally armed citizens when trying to help a police officer made the situation worse although I'm sure there must be at least one. But on the flip side there are numerous I say again numerous incidents throughout the years where a good guy with a gun has saved the police officers life.
Whenever I had a Ride Along with me who appeared to be a confident Able Bodied person although according to department regulation they were not allowed to carry their personal firearm (including my twin brother) I did however tell them how to access my locked Shotgun by showing where the hidden button was in case the situation went to **** in order to either help save my life or their own.
I hope that answers your question.