Do you have clients that ask for a "mobile only" campaign? In the old days, about a year ago it was no problem to create a paid search campaign for mobile only. Now, since the release of "enhanced campaigns" the topic gets a little trickier. It's easy to EXCLUDE mobile, but a little more difficult to exclude computers and tablets from pay per click.
As PPC consultants, we have experimented with several ways of skirting the issue. i.e. Bid super low on search terms and use enormous bid modifiers on mobile. For example, you have a search bid of $.05 and a mobile bid modifier of 500% — This in effect gives you a mobile bid of $.25, but a search PPC bid too low to generate impressions — Thus a mobile only campaign.
However, as the consultant, I do not recommend this strategy. It really limits your ability to hone in and optimize those conversions. If traffic is your only concern, then fine. If optimization and ROI are your goal, then bite the bullet and take/optimize all traffic sources. Having a true ppc specialist in your corner will likely make the venture more successful than you originally thought.