bacon is possibly the most UI embellishing stock art there is. Of course the design is good, there's bacon!+Andrew Kelly
not saying there's a simple right answer on Holo backport, but a 'pro' to throw into the equation is that Holo-esque apps on pre-Holo devices will prepare User expectations for the eventual move to Holo
(i.e. getting them onto Holo early dampens the shell-shock of moving to Holo later and potentially blaming OS inexperience on your app).
Holo aside, having a universal theme is a way to defeat a lot of the problems caused by old manufacturer variations to stock theme (e.g. dark buttons on Sony devices, where they're mostly otherwise not so dark on other devices).
Outside of the knowledge-base of Android developers and Android designers, others (e.g. managers, non-Android-savvy designers) can have a hard time dealing with the concept of "the UI doesn't always look the one way it was designed to look".