@Background Pony #A01E
I use upvotes to keep track of all images I like, so that I can easily return to them again later on. For this purpose it would not matter whether they were visible to others.
That said, I would enjoy being able to see the number of people watching a tag, and furthermore this information could be useful on a "top list" of sorts, as a way to find out what interests people most on Derpibooru.
Regarding artists and their tags, introducing the ability for them to see how many people watch their tags could be a step towards making Derpibooru a platform for artists on its own. The introduction of the artist badge a few months ago was a large, though perhaps unintentional step in this direction. It likely helped change some artists perception of Derpibooru as "that place where people upload my art without my permission" to a viable place for them to spread their art themselves. It already appears that some artists choose to publish their art first or exclusively on Derpibooru. More features and steps in this direction could put Derpibooru on par with deviantART and tumblr as publishing platforms for pony artists, with policies more permissive than most alternatives and potential new features allowing them to stay in close contact with their fans.
With all of that said, this is probably not what the Derpibooru staff has planned for the site and it is probably for the better. A site which manages the relation between artists and fans faces quite a few challenges as has been seen on other sites throughout the years, and it is probably outside the scope of a booru.
Still, introduce the watched tag count if you wish — making the site a little friendlier towards artists without taking on responsibility for large new features wouldn't hurt, I'm sure.
(You'll have to excuse my wavering opinion throughout the text — it is 5:30 AM and I need some sleep.)