This is very true. I think one of the biggest issues that exists within the band parent community is poor behavior at competitions. Unlike sports, these people (even sometimes 3 to 4 years or more into the activity) do not realize that these are competitions with entirely subjective outcomes. They think cheering louder for a band or not for another will impact the scoring.
Also, gossip about other schools is commonplace in the stands, usually involving less than accurate and sometimes downright misinformed/malicious comments being made among triggered 40-50somethings. Many times, jaded band parents or alumni who 'can't let go' have to find a way to take a hatchet to any band's success on the field. Now, there are entirely fair reasons to gripe about a band theoretically holding a competitive advantage over others' (combining multiple high schools into one group, etc), but in general, band parents can be nasty & territorial about the most ridiculous things.
One thing I believe that band parents should strive to understand is that, as they watch their groups grow over the season, not every band has the same goals throughout the season. Some finish earlier than others, are more complete at a given time than others, but unless it's the final competition of the season for all, most of the time direct compares are dangerous to make. Only one snapshot at a time.
I just wish more band parents and former band parents had the right mindset. I see a few on other forums (even one in particular on this thread) who are also alumni who 'can't let go' and it sickens me how they try to rule over online discourse in an activity that desperately needs diversity of thought and respect for all opinions to truly evolve.