Flash has obviously been very beneficial to them in the long run. It has given them the only remaining well-controlled proprietary piece of the web. This helps them sell their IDE, and more importantly, gets their brand out there.
Now, I'd argue that these goals have now been accomplished. Adobe is well-entrenched in web history, and everyone knows what Flash is. However, the relevance of Flash is clearly declining, due to HTML5, and stigma and disgruntlement is increasing. This means they will get less and less sales of their IDE and their name will fizzle out.
Imagine for a second that they open sourced the Flash player. Just the player. Suddenly it would no longer carry such a stigma with Linux, it would be easy to include in distros, developers would contribute fixes and make it more efficient on hard-to-support systems. It would literally stretch out its life-time as a product, and keep Adobe's name on the web.
I argue that Flash has played out its role for Adobe, and if they open source it now it could only benefit them. I did not think this was true in the past, and I think it will not be true in 5 to 10 years when HTML5 has surpassed Flash adoption in the most important venues. However, right now I think it would benefit them immensely.
There also seems to be a sentiment from some of the comments here that they are losing interest in maintaining Flash, so opening it to the community would seem to make some sense. If the "standard" ends up evolving in any way, they'd always have a head-start in their IDE support, since it will easily remain ahead of the curve.