Registrations are the single most powerful feature in an arranger keyboard. It's possible to have great amounts of enjoyment without using them, sure. You have OTS, Music Finder and now Playlists. But these are all just other people's ideas of how a certain tune should sound, or what goes best with a given rhythm. (And some OTS are great, some are dire. I've managed to get more than a few of the dud ones changed over the years!)
There's nothing to beat setting up and saving your own settings, even if you use OTS etc as a starting point. And then, to add variety into a tune, you need to change sounds, rhythm styles/variations etc while you're playing it. A tune played straight through with just one sound can be very dull.
Changing sounds/styles is mandatory in all keyboard exams, even from Grade 1, and all my students get taught how to to it using Registration Memories. If their keyboard hasn't got them, I get them to change it before they embark on the exam pieces!
If I take on a student with a higher end Yammie, the first question I ask is "Do you use registrations?" If the answer is "No" then the first lesson revolves around it. It makes no difference if they're 8 or 88, I make sure they know how registrations work.
And you need to practise using registrations and registration sequences just the same as you practise playing the notes and chords!