I've recently started attending the local church, since I'll be getting married there in about 18 months time. There's nothing particularly interesting about that fact, excepting that last night the sermon wasn't by the vicar, it was by a local music teacher. The subject was preparations for exams (apparently, it'll be the music exams this week), and she mentioned how often her pupils don't practice their scales and arpeggios.
I'm not a music teacher, and I have no plans to do it in any capacity, but if your pupils aren't learning their scales then something is wrong. That's usually because they don't understand what they're for so you spend time explaining how each piece of music is built up of scales and arpeggios of some description, and thus learning pieces is easier.
That is not why you should play scales.
It's technically true that scales are the building blocks of pieces, and knowing your scales does make playing easier (and certainly helps with improvisation - ask any half decent jazz musician to play a scale and they'll be able to do it, though they might not get why if they aren't classically trained). Scales and arpeggios are on the list of exam syllabus because they're easy to play.
A piece has a bunch of complicated things happening. There's a definitive melody. There's change in dynamics. There's the various techniques, like plucking the strings instead of bowing. All of these things add layers upon the raw musicianship that's required to do well in these exams - but they're usually shallow, and won't give the the real ability to learn how to play the instrument better.
It took me until last year to appreciate why I should be playing scales, and it's this: The notes of a scale are simple, so you don't need to devote time to looking at them and knowing what note to play next. It's just the next in the sequence. That means your brain has the chance to focus on learning the other things that make you sound better.
I have two scale exercises I'm using at the moment. The first is playing each note three times, going up and down, and progressively playing each note a bit faster. Why? Because I'm listening to a lot of Scottish stuff and I'm really enjoying the triplet thing that scottish guitarists do. I know that the barrier to playing that is my pick speed, so I'm practicing making those movements. Then I'll do it again, but alternate doing the triplets so I get practice at not playing triplets and then playing triplets - like I would in a tune.
The second is playing the scale in as many different ways as possible. I noticed at the last lesson I had that the barrier to interesting arrangements is that I don't know the guitar well enough. I'm quite good at the first 6 frets but beyond that I'm not entirely confident that if I have my finger on a fret at the 8th/9th position and play a string I'll know what note that is and how I can use it. That's been an evolution of the style I have, which isn't necessarily a bad thing, but the more tools I have in my toolbox the better I'll be.
If you're a parent and your child is learning an instrument, please ask their teacher to teach scales to them. Don't let them become a boring part of the exam syllabus. Let them spend a lesson playing a couple of scales, playing them in different ways. Let them start to appreciate why scales are so useful. If they get bored and want to move on to "fun stuff" then let them, but just 3-4 minutes every lesson playing a scale and talking about how to sound better is 3-4 minutes that's not wasted.