'server' mode in this context means that the whole page will be reloaded immediately on each selection change in tree - so, just to confirm, is this exactly what you need? If you set 'ajax' mode, server listeners will get events for selection changes.
Well, I use ajax mode now, together with the "render" attribute, and that works ok for me.
Maybe you want to implement "server" for the sake of completeness, like there is for the collapsable panel switch type,
but that depends on your priorities.
Anyway, the documentation should match the code :-)