A while back, someone asked me if I'd write a post here about studying nines. I do love me some nines, so I'm happy to oblige...
How often are nines playable? Not very often, really. As I understand it, about every 10-15 games or so, a position comes up where a nine-letter (or longer) bingo is both playable and the best play available. And many of those are just eights with a standard S plural or -ED and -ING forms of shorter verbs, which don't require studying nines specifically to know. You could probably win a Nationals or Worlds without playing a single nine-letter bingo, if you did everything else well enough.
The nines do have their uses, though. The ones that are unconventional hooks on eights, like INTERVALE or RELOCATEE, can be used to open up some doors for yourself, particularly when your opponent isn't likely to recognize the threat of the hook. Knowing nines can also give you some elegant extension possibilities, particularly when they hit a triple word square - NON(POETIC), COUCH(ETTE), EX(CHEQUER) and IMIN(AZOLE) are a few of the many I've seen on expert boards. It's rare you'll get to play one, but if you're thinking about those possibilities, that's a sign that you're really engaged in finding your best play - and that's a good habit to cultivate.
In terms of study priority, I'd rank the top couple of thousand nines on par with the medium-low probability eights. Neither category will help you that much, but if you've studied that far into the book, you're already contending with the law of diminishing returns either way. Nines that play through certain two-letter words are a little more useful, and it's a good habit to think of these as part of your board vision, particularly when you have a blank and lots of options - TI is a good one, for example, if you have words ending in -ATION. ER and RE, DE and ED are ripe for having nines played to or through them. (In Collins, CH# allows some beautiful nines every once in a while.) In a lot of situations where a nine-letter bingo is optimal, the nine is a double-double through a two-letter word in a double-double lane, so be attuned to that possibility.
But mostly, learning nines means you're a real word freak! That's not for everyone, I'll grant, but nines as a group of words are really fun to explore if you're into that. And studying them, while they're unlikely to be played, still has some value: they build your anagramming muscles. Remember when I talked about how eights were much tougher than sevens? Adding a ninth letter takes that up another big notch. The nines also contain a lot more compound words and longer word parts, making the anagramming task much more complex - and satisfying, when you solve the puzzles. You do enough of those, and eights start looking EASY. And oh my god, if you ever did get (BA)HUVRIHI down...okay, maybe it's just another bingo. 76 points is 76 points. But if you look at the track records of the very best players, about all of them have a body of work that includes a few sweet long-word finds like SAPROZOIC, SATINWOODS, AUBERGINES or WATERZOOI, and that's not an accident. It's an indicator of the desire these players have to master the whole game, including the furthest edges of the word list.
I do have to warn of a certain disease, though: just because you see a nine doesn't mean it's the best play. Sometimes it's not, and sorry, tournaments don't give out style points, so take the best play and just be happy you saw I(TI)NERANT for fewer points and worse board position than ENTRAIN elsewhere.