There are plenty of things that people can (and do!) pay extra for. The Segway ride at Epcot is one of them that comes to mind.
Personally, I think that putting the bulk of the theme park product into one fixed price is smart -- not to mention it's the way that all theme parks I have visited since the end of the ticketed ride system went away at Disney do things.
I guess the question is what do you consider an "extra". Tours are no-brainers as "extras", as are special-access things like IllumiNations cruises. I would not like to see an actual attraction in the park that has an additional charge. At my last visit, Six Flags near Atlanta had a bungee-swing thing in it that was an additional charge, and I felt that it was an actual attraction (because it was a ride), so charging extra for it was kind of tacky. But people sure were lining up to do it, and as long as people will pay, that sort of thing will happen.
Personally, all rides and shows like the fireworks should be included in the base price. Additional stuff (like arcade games or games of chance where you win a stuffed animal) being an additional charge make sense to me... especially since the ring-toss or whatever can provide you something tangible that you walk away with.