curtisp.com
Many argue that Flash is unstable. To be sure, most of this is anecdotal at this point, but I think that's true. For a device that emphasises browsing, stability while doing so is a must. If mobile Safari crashes, Apple cop the blame. So that's the first reason for excluding it.
It is slow. It's a battery hog. Simple as that. It's completely unsuited to mobile devices.
So think about it, given those reasons, how would a boycott help? From their point of view you're asking them to make the browsing experience worse. What needs to be done to get them to change their mind? What can Adobe do?
Basically a boycott at this point is like saying "I know you think it's crap, but just do it anyway". It's making a demand without an actual argument to back it up.
arbitrary adjective based on random choice or personal whim, rather than any reason or system
With the automatic bug report generation in OS X, Apple has direct evidence of Flash stability problems, so their knowledge of instability is NOT anecdotal either.
anecdotal| adjective (of an account) not necessarily true or reliable, because based on personal accounts rather than facts or research
Stability problems go hand in hand with security problems. A search of the major security sites will produce a long history of vulnerabilities and also demonstrate that fixes have been slow in coming.
And on an individual level most of us can likely see an example of the instability on any platform by visiting http://flashcrash.dempsky.org/
(The 10.1 beta, which is not in any OS/browser by default, is reported to have a fix. Of course disabling Flash from loading with NoScript, Flash Block etc avoids the crash also)