You're getting your fine muscle control (writing with a pen) mixed up with your grip. Grab a bar with every finger but your pinky, then do it again with your thumb left out. I mean I don't know what to tell you if you're going to let your personal opinion and your not so scientific method of observations determine your answer but I don't think you're really going to find a source on the internet telling you otherwise.
http://mashable.com/2014/07/06/pinky-finger-phone/
http://www.nytimes.com/2008/12/16/health/16pink.html?_r=0
http://www.amazon.com/Your-Pinkie-More-Powerful-Thumb/dp/0062008358
Those are just a few articles, basically you can google most important finger, and it's not going to be the thumb. I mean this is some classic mansplaining right here, so I know nothing I can say is probably going to change your mind. But you're wrong. How you hold your tools with the use of all five of your fingers is not how you determine the importance of a single digit.