There is a thread for this already, but it is locked and it doesn't express a convincing reason (to me) why this is important.
English and many Latin based languages are sinistrodextral languages, they start on the left. Our eyes must locate this position to start reading, thus anything that displaces this position from its most common location adds burden to the reader.
The sidebar is a great feature for optional content and it is naturally secondary to the main content. I don't see the reason why secondary content should displace the main content, instead of just limit its extension by opening at its end (right).
The only exception that makes sense to me is navigation menus, like bookmark panels, which have a direct influence on what the main content is. In some cases, the sidebar can act like this, but it doesn't seem to be its main purpose (for what i've seen).
As a new user of Opera, this and having single-website bookmark icons on the sidebar, is the only two features i'm missing for a perfect browsing experience.