Yes! when I imported all the available image tags information into my relational database and ran a distinct query for tag names, it gave me 654 values! I figured that adding "-" would cost a lot of space to the database and would not be a viable option. I guess, XML would be a better option to store all the tags.
Guess, that means -T and -All option should not be used together.
Digressing, I would like to add that exiftool is one of the most wonderful tools that I have come across (not just w.r.t Meta Information softwares). It is one of those few tools that makes you just instantly fall in love. Kudos to you!