I wish I could say this was a surprise: of the security issues that Apple patched in Mac OS X 10.6.5, 42% were to deal with Adobe Flash.
The security issues related to Flash are in fact the stated reason why Apple is backing away from bundling the plugin with its new computers. Apple began shipping the MacBook Air without Flash installed, noting that customers could install the plugin on their own to ensure they had the latest, most secure version.
However, testing indicates that in normal operation, Flash can also consume dramatic amounts of battery life just to animate web ads in the background, resulting in as much as two hours of lost productivity on a single charge.