I am for it as long as it does not change their functionality away from being able to show ebooks (or installing a complete multi purpose operating system).
This means jailbreaking it to add functionality to read other formats, have better [pdf] viewers, allow more advanced syncing options or other encryption standards for wifi connections should be included.
Also tools like journal apps, clocks, music players or reminders should definitely be allowed. And so should be operating systems like duokan for kindle, which is just another e-reader interface.
I do however not really regard installing Ubuntu or Android on the reader as on topic (I am not speaking of them already being installed in the first place). But I would like applications to be, that allow i.e. to log into the existing OS to change necessary parameters not accessible from the user interface .