Yes, it's useless until...
* They need to read someone else's cursive handwriting and can't (if you've not got the experience of it, it just looks like a mess)
* Their battery runs out and they need to write something down
* They need to scribble down a note during a phone call
* They need to fill out a prescribed form
* They need to write out a calculation (not that you can't do that on a computer, but loading up MathCAD to do some relatively simple maths- or spending a while crafting an Excel spreadsheet to do the same is nowhere near as simple (actually, the iPad doesn't HAVE software to do more advanced maths, does it?!)).
* They need to draw a graph.
* They need to annotate someone else's scribbles
* They want to pass notes at school
* They're sitting an exam, which should be away from the computer to prevent cheating (can't guarantee no network/internet connectivity) and to encourage their brains to do the thinking for them (i.e. they do the maths, not their Calculator app).
Or any other of a million and one tasks that require handwriting. I'd also bet it improves fine motor skills and co-ordination when taught from an early age.
So yes, after a couple of years it's probably okay to not teach it again. But it's definitely a required skill.