Your question has really perplexed me...i think it would have been easier to tell you a concert where anyone WOULD actually want to see all the bands!!! I think the only case that I've experienced in which i wanted to see opener and headliner was back when a perfect circle opened for nin. it's funny you mentioned the zombie/ozzy combo because at ozzfest several years back we watched rob then left like 2 songs into ozzy's set because it was Boring-and we figured we might as well beat the traffic. (had it been black sabbath maybe we would've stayed). Anyway, the only concert coming to my area that i remotely want to see is Helmet, and fortunately, it looks as though they're solo! When you don't want to see the headliner i think it's a better scenario though because you can just leave, as opposed to waiting around through crappy openers to secure a spot to see the main act.
i thoroughly researched all the concerts coming to my area, and Nothing of any value!!! (unless you want to see jonathan davis of korn-solo, what's that all about?)