This first came up when SE introduced displaying the most common tag at the start page s for SEO reasons.

Commenting on the following two questions and their new titles

• ubuntu desktop - What native games are available? - Ask Ubuntu - Stack Exchange
• ubuntu desktop - How do I make my purchased music be synchronized on Rhythmbox and in ~./ubuntuone/Purchased from Ubuntu One? - Ask Ubuntu - Stack Exchange

I said:

The ubuntu-desktop is the most used tagged on the site, but is no doubt something very unlikely to be entered by users into a search engine, and therefore adds nothing for SEO. (Note I'm not saying I think we should stop using this tag.)

Jeff Atwood's response included:

I would also argue that this encourages proper and correct tagging of questions. If the tag is objectionable in the title, then it's probably a bad tag practice -- as you pointed out!

I wanted to seek the community's views on this.