I can understand having the interface consistent to simplify guiding
newbies—"click the little button in the upper-right corner with the X on
it..."—but as for poorly written themes, that's a bad excuse. They let
people install poorly written software which is much more dangerous.
Imagine the outrage if they started limiting the kind of software you could
use.
I also don't understand why StyleXP, WindowBlinds, etc. use the MS
visual styles to do their thing. Why didn't they just come up with their
own implementations. Obviously the MS method was good enough to support
anything people want to do when it comes to visual styles. Microsoft chose
to add visual styles to Windows, preventing people from using anything but
MS themes is a poor move and was quickly defeated. Their implementation
while belated as it was—people were expecting an official version for quite
some time and had to resort to 3rd party utilities in the meantime—would
have been lauded and appreciated had MS opened it to everyone. But since
they didn't, people just saw it as a neat little thing for a few days until
you get tired of the same 3 styles. That's why Stardock and TGTSoft made an
easy killing once ThemeUI.dll was cracked. Microsoft really needs good PR
and allowing 3rd party themes really could have given them a much needed
boost.