Would you like a broom with that statement? I disagree with this. Like you, I take the boy scout attitude of "be prepared" and go to gigs with extra basses, leads, amp heads, stings etc... because I am a pro. The pros I play with are the same and cover themselves and others. We have saved each other's skins and gigs many times and enjoy doing so. I find it's those that think they are pro that don't actually behave as a pro. Yes there are exceptions, and I'll conceded that.
I've just started a big pro theatre gig and am joyous in the professionalism of the group and crew. I found it difficult to stay out of the way and not help as the crew are so competent. I just show up with my basses, pedal board, leads and IEM and off we go. Lots of great compliments from returning fans so it must be working.