Built in, like IE was? (integrated into the OS to make sure you couldn't get rid of it without affecting the performance of the OS.) Built in as in just integrated? or built in as in "included with"?
While I seriously doubt that the new 3D paint will be able to take on the likes of ZBrush, Maya, or 3Dmax (or even Blender) ... We don't need bloat or added vulnerabilities due to "built-in" software. I'd rather they be optional, non-detrimental if not installed, and not pushed as if it was as important as a security update or bug fix.
Other than that, it may be pretty interesting to see what 3D Paint *can* and *cannot* actually do. For some the Game mode may be a welcome addition even.