I see your point here.
I can add up the casting times + GCD's and get a rough figure of my reset values (then change these values every patch or talent that alters cast time, etc).
My player unitframe has a cast bar, but I cant see the cast bar from my alts.

Though by no means necessary a timer would be a useful tool.
(unless I can get an addon that shows me all 5 castbars)