That's an additional device, additional financing potentially on a different life cycle, potentially a different account to manage.
For a dedicated iOS dev that's worth the hassle, for someone managing a team of devs or employees only needing apps to run without requiring a full dev env, it's quite convenient and potentially cheaper.
Not for fast user interaction tests.
I have a Linux laptop with touchscreen, and my main use is to test the touch interactions on web apps, sometimes Android apps.
Sure it's possible to open it on the phone, but that's an extra step usually not justified.
That's an additional device, additional financing potentially on a different life cycle, potentially a different account to manage.
For a dedicated iOS dev that's worth the hassle, for someone managing a team of devs or employees only needing apps to run without requiring a full dev env, it's quite convenient and potentially cheaper.
I don't own an iphone
You could buy one from the price difference of the touch MacBook.