RMS and other GNU developers used non-free software in the beginning because there was no way to solve the circular chicken-and-egg problem.
Surely no circularity here: How did the used non-free software come about? 50 years before rms no software existed, and then it was written (and proprietary). rms could have written gcc from scratch just the same.
He was right in not doing so NOT because he "had no choice" (he had), but because he maximised the freedom he brought.
The same can be said to a limited extent for other essential packages, too.
"Essential" in the sense of necessary for a hacker when programming. But essential to a common user, potentially committed to a life without writing one line of code, is something totally different.
Flash might well be essential in the sense that without it, no switch to a freer system will occur.
For the sake of concreteness, when you can help a friend switch from {WinXP, MS Office 2003, Outlook, IE, adobe flash} to {GNU/Linux, OOo, Thunderbird, Firefox, adobe flash} but *not* to {GNU/Linux, OOo, Thunderbird, Firefox, Gnash} because he'd rather keep his original system, then I think it's laudable to do so!
You are right that he might get even more dependent on non-free flash. But he would do so anyway! And if we refuse to help reaching this intermediate step, he will *additionally* become dependent on docx and Vista (and I don't know what dirty things Outlook and IE have in store).
We all agree we prefer the third set over the second. It's a pity we can't agree to prefer the second over the first.
Freedom is not binary. And the more freedom the better.
!hwe
PS: It goes without saying that the correct version of set 2 is {GNU/Linux, OOo, Thunderbird, Firefox, adobe flash + explanation of the issue}.