@Brijesh, brother even I am stunned after reading to what you quoted. I had not thought about the
rel='tag' so deeply ever before than today.
So, firstly let me tell you that when and why should you use.
#When Should You Use
You should not just blindly add
rel='tag' to any URL, which is of a label page. It does not make any sense.
Rel='tag' tag was originally introduced to indicate the page where a major portion of the post is present, and similar other articles are located (that’s what the label page for Blogger users is).
Now, you are asking why MBT is not using
rel='tag' in the navigation. Let me explain it you by taking the example of @Mohammad’s latest post on MBT:
Best Yoast Alternative
Now, as you can undoubtedly see that the main label (set by the author) is “SEO, WordPress.” So, the tags should be only “SEO” and “WordPress” not “Make Money”, “Social Media”, “Widgets” and “Blogging” (the labels on MBT’s navigation). I hope you have understood now. The links in which
rel='tag' should be placed should be related to the post that the user is reading. You may not put
rel='tag' on all the labels you find on a page.
By the way, I don’t think you would be penalized if you won’t use this tag because these days, bots are smart enough, and they can easily understand that which post is the collection of same labeled posts and which is an entire post page. [As far as I think]