Pro: It aids searching
You can search by a tag, and you could not necessarily search for the same concept without the tag. There are various ways that the same answer-requirement could be included in a question’s body, not all of which would appear from the same search.
Pro: It indicates an important aspect of the question to those browsing the list of questions
You can see tags from the list of questions. The rules-as-written tag on the homepage tells you something useful about the question without having to go inside it. This allows users to decide if the question meets their interests/is worth their time/etc.
Pro: It can be followed or ignored
The tag, rather than merely having it as part of the question-body, also allows RAW questions to be followed (as I have) or ignored (as others have stated they have).
Pro: It reflects a common pattern of at least one major RPG community that we support
D&D 3.5 questions habitually refer to RAW or not-necessarily-RAW as a matter of course. In systems less sophisticated than ours, threads are marked with “[RAW]” or whatever to indicate this. Considering that RAW is used as a tag even when the underlying system does not support tags, it’s at the very least worth considering its use as a tag here on those grounds alone.
Neutral comment: Not all systems or communities make the distinction
This isn’t really a con; after all, we have plenty of tags relating to specific concepts in various systems that don’t really work for other systems. It may be the case that many other systems have no use for the tag, but that does not, in itself, invalidate the tag.
Con: It is “meta”
I am not the one to ask about the “problems” of meta tags, because in my opinion, those problems are minimal and frequently overstated. I don’t dispute that the tag is meta; I merely don’t particularly care. I don’t have a problem with meta tags, provided they are useful, and I contend that this one is.
As for being used on its own, we more-or-less require that every question use a system tag: questions without a system tag are expected to use system-agnostic. It may be more or less dependent than other tags, but I hardly think it unique in that regard.
Con: We just had a kerfuffle over it
I had a knee-jerk reaction as I was heading out the door, and I regret it. It was not the appropriate response to the situation. Ultimately, it was an issue of treating the tags on the question as all being required, rather than the intent, which was that answers from D&D 3.5 or Pathfinder be acceptable so long as they were indicated. I would argue that this was a poor choice in the initial tagging since the rules-as-written are not identical in this case, and either-or means that a valid answer could ignore one of the systems it was tagged as, but that’s irrelevant to my actions (I should have made that argument rather than just acting).
I really don’t think that a bigger issue needs be made of this than already has been.