Every musical artist has to start somewhere. Your favorite band was once just an opener for someone else. Why wouldn’t you give an opening band the same respect that you would a headliner? Why would you spend money on a ticket to see three bands, yet only listen to one?
Some of my favorite bands at the moment were bands that I saw open for a different band. I love Saint Motel because I saw them open for Panic! At The Disco. I love Judah and the Lion, Chef'Special, Jon Bellion, and Vesperteen because I saw them open for Twenty One Pilots. I love Bonelang because I saw them open for Vesperteen. If you take the time to listen to the opening act, you may leave the venue with a few new albums to check out.
As an opening band, you have a lot of pressure on you. You want to make sure that the audience enjoys it and that the headlining band doesn't regret bringing you on tour with them. If a band does do well as an opener, it could be impossible for them to get better gigs later on. The entire experience is sort of a "trial run" to see if they could handle their own crowds.
I’m not saying that you have to like an opening band. Sometimes they aren’t playing a genre you like. Sometimes you don’t like the lead singer’s voice. Maybe it’s too loud for you. That’s fine! I just hope that you can be respectful enough to not bad mouth them out loud while they’re performing a few feet away. You never know who could be listening.