My reasoning for why I will bitch when there is shit support for a band I'd like to see is its harder for me to want to shell out $30+ to just see the headliner. I have a hard time wanting to see a band play 70-90 minutes and then leave. If I shell out $ for the ticket and any other concert-related expenses I, personally, don't want to show up for just a headlining band and leave. I'd rather spend the $ on a whole concert I'd want to see.
Granted, if the headliner is someone I REALLY want to see and its not a long drive then yea I'll show up late. But the majority of the time I'd much rather see a whole show I want to see. Maybe some feel this way, maybe some don't.