The new viewer uses a configurable toolbar now, so why not same the for Metapanel (e.g. to additionally "fill" it with tagging tools, rename-presets, etc.)?
The viewer has a toolbar because there wouldn't be a toolbar there otherwise.
The metadata panel usually exists in the lister or the viewer, where you already have toolbars to put those buttons in.
(Although there is a standalone metadata window, I bet most people don't even find it. 
)
I'm not against the idea, but it's also less necessary than with the viewer, and more a question of where things are in the window than whether or not it's possible to put them in the window at all.