Just my 0.02$ based on forum threads reviews. I'm almost sure that we have it to be turned on by default as
- too much people asked to implement it when we have no such feature.
- many people asks at forum "how to make my buttons looks the same as at rf-demo". (it off by default in 3.3.x)
- just a few posts asks how to turn it off(in case it was added just by copy paste from samples web.xml's).
And I believe that's one of the core skinning mechanism features which makes application has Rich and smooth design with less efforts. So I'm vote for "yes"
I too vote for yes, it will still be easy to turn off if users wish.