When I want to select items in my presentation, those items are suddenly moved by accident as the “big button” thinks I’m using the Touchpad, but I’m using the Trackpoint. I often create Powerpoint-Presentations for my talks in the train as I travel quite a lot. Also you need to move your thumb very far to the left/right to do a left/right-click. This happens because you only have that single big Button.
50% of your left-clicks are a middle-click by accident. I own a W540 and as a hardcore-Trackpoint-user it’s really a pain not to have the hardware-buttons. Lenovo has replaced the hardware-buttons for the Trackpoint with a big Touchpad.