Having a button on either side is a horrible design. I could see accidentally pressing the far button with my ring finger or pinkie all of the time with that kind of configuration, not to mention not having the ease and natural feeling of movement of being able to roll my thumb across the two buttons.
I use my side buttons in many applications. In my browser, it's forward and back a page. In my audio and video players they are used to skip forward and backward 10 seconds. In my text/code editor, they are used for page up and page down. In games they are used for jump and crouch. Having one button on either side would be a usability nightmare.
You can always disable them or assign them to do nothing. It's a 2 minutes activity. My gripe with these kind of mice is that they advert the full number of buttons that they have on the mouse without mentioning that a couple of them are practically unusable.