Create a report that segments devices and Top vs Other to see what the difference in click-through-rate is for tablets and smartphones when your ad isn’t in the top spot.
You’ll probably find that click through rate drops below 1% when your ad isn’t in the top position for tablets and smartphones, so by having mobile-only and tablet-only campaigns, you can better optimize bids for those device specific positions.
Another reason to make mobile-only and tablet-only campaigns different from desktop targeted campaigns is to write adtext tailored to the devices, something like “…Shop Now From Your Tablet!” or “…Find A Store Near You On Your Phone.”
Google has ad extensions for mobile phones like location extensions and call extensions that boost click through rates and make mobile search ads very effective at driving in store traffic. Optimizing and reporting is easier when you have a mobile-only campaign.