Nope, or at least, you shouldn't be... my middle button is a click-able scroll wheel too. Only thing I notice is that I need to be quite precise when using the wheel as a middle button...
I need to place the mouse over the 'to be clicked on' item, and then press down quite firmly for the click to register. I figured it was just my mouse... maybe not though, maybe yours too?
drifting slightly off topic:
This is my second Logitech mouse. My first one was an MX 1000. It was a nice, solid mouse that I liked a lot... untill it started firing double clicks when I was only single clicking. It got worse and worse untill it got so bad the mouse was unuseable. I got about 18 months out of that mouse. I was not impressed, but figured I'd just got a lemon. It's the kind of thing that happens to me a lot... I can be presented with shelves, stacked floor to ceiling with one thing or another, and you can put your savings on me picking up the one box with a critical piece missing!!!!
Here I am, with a 19 month old MX Revolution, and yeah, it's started firing of the odd double click when I'm only single clicking. What do you think, Logitech in general, or just my bad luck?
-marko.