A guide is ment to be follow up to the point it can offer, you should also need to have some criteria and see which playstyle is better suited for you.
boss guides depends on particular bosses mechanics, guild strategies, difficulty, etc. If blizz decides to change X mechanic, you should rethink using a guide having that in mind. Same case with PTR mythic and live. Guide will tell you about the general aspect of PTR, but it could have change anytime... Was it usefull? yes, for a starter, then it will depend on how the guild will aproach the fight and deal with some mechanics.
So for someone who hasnt done PTR and its aproaching the mythic encounter for the first time, it is usefull. Now, it can change, or it wont. It can have a different strat, but anyway it gives you a tool for having that first contact, until it all gets more "mainstreamed". Either way, it is a great contribution, and people should follow it as any other informative source, under the criteria of what you decide to do with it and which other sources you decide to follow.
tl;dr : memespec all fights