With the advent of ultraportable laptops a few years ago, I've almost entirely stopped using my tablet(s). The upsides of a smartphone's form factor are self-evident, but a suitably light laptop is (IME) so far superior to a tablet in comfort, usability, and efficiency that the category has been practically obsoleted for me.
My current laptop has a 13 inch screen and roughly the dimensions of an 11 inch macbook air. There are very few times I find it less convenient than a 7 inch tablet, and almost none compared to a full-sized tablet.
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.
I tried doing some serious writing on an iPad (with external keyboard) a few years back.
The number of times I needed to touch the screen or switch apps made it a pretty inefficient experience.
If I wanted a 'cheap' device to write on again I'd probably try a Chromebook
In my experience my iPad means I am using my laptop a lot less, but it's taking me more time to do the same things I did on the laptop on my iPad.