Not too put too strong a bias on it, but: XML processors are a dime a dozen. There is no way for us to know *now* what the "best" XML processor(s) will be a decade from now, and Perl 6 is intended to be a very long term language. And frankly there are enough different use cases to ensure that no single XML processor could possibly be "best" in all circumstances anyway. We should not canonize a single XML processor (now especially) by putting it in the core.
As Nicholas pointed out, it's unlikely that "vanilla" will be the Perl 6 flavor that any vendor actually ships. But I definitely want to be able to choose between strawberry and chocolate, and perhaps a new flavor of my own (or my company's) design. I really do not want to always get "Baskin-Robbins in a blender" because everything's in core. The grammar engine is core. A *particular* grammar is not. -'f