For better or worse those crappy ads pay for the sites you are most likely looking at. Not sure what is worse ... ads built into applications where your privacy info is "phoned home" to remote site or flash embedded into website which try to do the same. The latter you can possibly stop if concerned. The former is much more insidious.
For better or worse some sites require flash to look at content. Often these are sites which as concerned with folks downloading their content without permission, but sometimes sites that just want more disco in their presentations. or just games.
Asking all of them to flip over to bleeding edge HTML5/Javascript isn't very pragmatic. It is like asking folks to dump all that IE5/6 specific crap in their websites. For example, Oracle just flipped the major portion of their support site into being Flash based. ( dubious or not it is in motion now.)
As far as crashes go.... yeah it is a problem but that is why browsers have taken to doing more to sandbox the plug-ins so that when they crap out it is just the browser (or minor subwindow of browser) that dies.
The major blocker to Flash on the iPhone was that it needed too much horsepower ( CPU and battery). Whether that is fixed on the tablet/slate/pad is open to question. Certainly could have bigger battery and should have substantially faster CPU. However, those resources may be thrown at other software or issues.
However, flash would be a differentiator between iPhone/Touch. It is on track to show up on Android later this year. Think WebOS too. If Apple steps past better have a good story to go along with it.
Many "for pay" sites would disagree. You can block all of the "script kiddie" simple mechanisms for remote cloning a site by going to a flash setup. With generic HTML mechanisms utlilites like wget and curl can clone a website with simple flick of command line construct.
