There may be value in using an iPad as a dev machine simply because it provides a separate environment for only doing dev work. Given that habit (and thereby procrastination) depends greatly on environment, it's possible that using an iPad strictly as a dev machine could increase productivity by subconsciously associating the iPad environment with efficiency. That is, if you're on your iPad, you know you shouldn't be messing around, and you use it only for work. Whereas if you're on your laptop you might be tempted to lapse into time sinks that you already associate with your laptop.