Block works best where there are small-but-fast hitting bosses, such as most of Naxx. With enough Block Value, you used to be able to make the Loatheb fight next to trivial as you'd block almost all of his incoming damage.
However, with Ulduar and now Crusaders' Coliseum, we're not seeing that kind of fight any more, so the gearing strategy has changed accordingly. Even with a lot of Block Value, you might end up shaving off 2.5k from a 27k hit from bosses like Hodir. While a guaranteed 10% mitigation isn't to be sniffed at, the reality is that it has very little impact on your time to live - after all, two full 27k swings will kill you if you get no heals in exactly the same way that two blocked 24.5k swings will. In short, it doesn't do enough to save you most of the time, whereas avoidance can.
I disagree with your assumption about it being a tank's job to get hit though - it's the tank's job to maintain aggro and stay alive, getting hit is just a byproduct of that. You can maintain your mana through dodges, misses or parries through Blessing of Sanctuary.
If at all possible, having a "block set" for trash or instances you outgear is a great idea but for progression main tanking, you want a big pool of effective health (Stam+Armor) then as much avoidance (miss+dodge+parry) as you can get to hopefully buy you some time where you don't take any damage, allowing the healers to keep you topped up.
It's a shame because 100% block uptime is one of the things that makes us Paladins unique just now, but we're simply not given the opportunity for it to be spectacularly useful post-3.1 as it will rarely be enough to save your life. It's very nice to be able to guarantee that you'll never take a full hit, don't get me wrong, but it'd be a mistake in terms of your overall TTL to invest heavily into block on your MT set.
Hope that helps