Did he issue you a ticket? If so, then look up Utah's laws, you might be in the right and could fight it.

I'm guessing he didn't cite you, likely, he just wanted to give advice for that particular road. You could write it off as a cop that's really worried about you or just doesn't like cyclists.
He might have good point; I'd agree with the cop that there are roads where "impeding" traffic is legal, but just bad idea (even though as you pointed out, you might be legally part of the traffic flow). Holding up traffic in name of the law is bad PR for cyclists, IMO, because it's sparks road rage, etc., even though some purists will contend that laws should be followed to the letter.

Regarding mirrors, they likely aren't required (I dunno Utah - Colorado doesn't require) but not a bad idea. Personally, I think a mirror is a better piece of safety equipment than a helmet.

Let the flaming begin...