Another favourite hero of mine, not because he's flashy or super fun to play with, but because he's just solid and dependable and reasonably cheap in threat. Balin is of that type of heroes sometimes called 'glue' heroes, in that they're not the focus of the deck, but they fill in the gaps and hold it all together. He's also of the type of hero that can simplify the deck-building process because his ability covers an aspect of the game you might otherwise need to devote some cards to - in this case obviously shadow cancellation.
Let's look at him in that aspect first. Now shadow cancellation is an effect which has variable significance. Often people will just leave it out of decks, favouring more powerful defence over actually cancelling shadow effects, so in those cases Balin obviously doesn't impact your deckbuilding. In cases where you're really concerned about shadow effects, it's possible you may consider Balin to be not enough, though the main case in which I'd find Balin insufficient wouldn't be cases of quests with worse shadow effects, rather it would be cases with potentially multiple shadow cards per enemy that would cause the problem. So aside from those, in cases where shadows are a concern I consider Balin generally enough and thus he saves on deckspace. And in those occasions where shadows aren't so much of a concern? Well in those cases, even if you feel like in general shadows shouldn't be that worrying, still it's very possible to have just an occasional shadow that really wrecks you, in which case there is Balin as your safety net to just smooth things out. It's exceedingly rare in my experience that he replaces the shadow with a worse shadow, because you tend to save him for pretty bad shadows in the first place. Besides which of course he will still function in his significant aspect as a glue hero.
And so now let's look at that. General traits of a glue hero: allow access to an important sphere and help with some important aspects of the game while having decent stats and a manageable threat cost so they just unobtrusively get on with making sure everything runs smoothly. I've already brought up of course how Balin's shadow cancellation can help smooth things out where you might otherwise have a rougher experience. 9 threat is indeed manageable, and it's not like you'll get much lower in Leadership. Leadership is a significant sphere to get access to for Steward of Gondor and Sneak Attack most prominently. Finally there's the fact that Balin is that great rarity - a Dwarf hero who not only doesn't depend on you playing Dwarves, but also doesn't even need you to really build around him at all. I mean seriously, look at the Dwarf heroes: Glóin requires healing to function consistently; Gimli ideally wants boosts; Thalin is only for if you want to build around direct damage and doesn't contribute much else; Dwalin deals with threat reduction in the right quests but you'll probably want some boosts for him; Dain Ironfoot, Thorin Oakenshield, Nori, Ori, Óin and Bombur only work if you're specifically playing a Dwarf deck; and Dori is an odd duck that really probably only fits a certain type of deck. That leaves us with Balin and Bifur as the only two Dwarf heroes who don't require you to build around them. The reason this is significant is of course King Under the Mountain, that phenomenal Dwarven card draw engine, and Balin slightly edges out Bifur in this regard because he sphere-matches it. Being able to drop King Under the Mountain into just about any deck is well worth it in my book. If Balin didn't exist, I think it's entirely possible we would then see Gloin being dropped into decks just to get access to KUtM without actually bothering about his ability.
So in brief, aside from his sometimes handy ability, solid stats (2 is good enough, 2 means he can maybe take a hit if needed, and 4 is enough to live through some archery or a weak undefended attack) and reasonably low threat, by being a Dwarf, Balin provides you with both resource generation and card draw at a single stroke. That, my friends, is serious value. He may not make the big flashy plays, but this guy will win you games.