RepeatButton not inheriting style of ButtonBase

In the January CTP, I have a style declared for ButtonBase. Later, in a
style declared for my scrollbar, I use a <RepeatButton>, but it will not
look like I tell a ButtonBase to look unless I explicitly say
Style="{StaticResource {x:Type ButtonBase}}" . RepeatButton is a subclass
of ButtonBase, so I would expect it to take on the background, etc. (and my
regular Button classes do work this way, although I explicitly have a button
style declared). Is what I am seeing a bug or by design such that concrete
classes will not implicitly inherit style from an abstract superclass?