but why not allow this new protocol to work correctly out of box?
One can see several reasons for that:
-if implementing the said protocol in cis "out of the box" (while it is, as you observed it yourself, very easy to do manually), why not do the same for other not cis-implemented windows protocols: remote desktop, telnet, iis, why not gopher, it maybe still is used by 3 persons in the world?
-llmnr is somewhat outdated: small business networks needing local communications (and not using a system administrator) have used concurrent solutions available far before llmnr and, a contrario, the "small lan user" with only a few computers has always been told by every software editor to depend only upon himself.
-A significative proportion of cis home users does not run a lan, and even if doing so, does not want these lan computers to communicate with each other.
-the free cis version is not supposed, if i remember well, to be installed on more then 3 computers: maybe the paid one achieves what you wish, i have no hint about this question.
-such an hypothesis therefore not only should be an option (some users would not want it by default), but also would lead cis to publish specific versions for xp, vista/7....: one sure thing is that, as a xp user, i don't want to hear about whatever broadcast or multicast service being allowed, and if possible not even about such a dedicated service to run.