My view is that the rules tag is a meta tag and is mostly unnecessary.
Here is the "official" stance on meta tags, straight from the horse's mouth.
Is it useful?
I can empathize with the people who propose that it's useful to have MTG questions tagged with rules so that it helps them categorize the question mentally at first glance. But how useful is it?
My view is that since we encourage users to ask questions in natural grammar (not search engine grammar), it's easy to understand that a question is about the rules of a game if a user reads the question carefully. If a user didn't read the question, then perhaps the topic is irrelevant to begin with and a rules tag would be equally irrelevant.
Also, the Follower-to-Question ratio of rules is 0.0102. It has 5 followers despite being the most popular tag for questions. That suggests that most users don't find it useful.
It'd also be interesting to see (but I can't) how many users have ignored the tag.
How do I use it?
If I were to follow the rules tag, wouldn't that suggest that I'm either 1) interested in learning about any rules from any game, or 2) knowledgable about any rules from any game?
I think either scenario is rare. A question tagged rules would ideally be only about rules, independent of any game.
I can also see the usefulness of a tag in that situation, even if it's rarely used, because it won't be used as a meta tag. But the current use of rules is mostly generating noise.