[JasuM]

Boost has a very liberal license, unlike Cairo. C++ libraries, at least the GNU C++ library, have directly based some of their implementations on Boost.

This might lead to C++ library vendors not implementing the standard and requiring using a third party library, perhaps later integrating them into their own libraries.

Thinking about it, it is not probably a bad thing, it will just take longer than with other parts of the standard. Of course, this is just pure speculation and things might turn out very differently.

[pmelendez]

You have a point if implementers choose to use cairo code instead of re-implementer it, but in the other case they don't need to follow any license at all since licenses are valid just for code, unless they have patents on it (which I don't think is the case in libcairo)