I know what it can be used for, but my point remains - nobody has felt the need to include an app in a/the mouse driver to 'helpfully' tell people on the systray "dude, you're using the mouse right now!".
I haven't checked with every touchpad driver, but AFAIK most can be configured to auto-disable the touchpad when a mouse has been plugged in, and otherwise, once you've got the touchpad configured the way you want it, are you honestly diving into its settings so often as to merit a systray app?
I just think that having it on by default should have gone out of fashion at least ten years ago.  If there is someone out there who wants/needs a systray icon for the touchpad, the option could remain to enable it, but otherwise it has to one of the most ignored systray icons, possibly second to the QuickTime player icon.