Because with this you can use Sublime Text's UI (chrome, file browser, and smooth scrolling—all better than Vim's) with Vim's input mode. Best of both worlds.
The point of vim and its key bindings is to not have to use anything other than the keyboard (and keys that are not too off from the home row), so it's not the best of both worlds at all. Having to click on something forces someone to use their mouse, breaking their concentration and wasting time on clicking on a tab.
Having to click on something is different from being able to click on something. Vim doesn't let you use one of the two primary input tools of your computer--Sublime lets you use both.
To borrow your situation, if you are already using the mouse it's distracting to have to use the keyboard.
Maybe you should actually use something before forming an opinion on it. Your lack of knowledge with vim is pretty obvious given that you didn't know that you could use your mouse.
