The short answer is, that Yes you can use block as main defense, but you will have to settle with dying some of the times - if you dont want that. It is a no go
It’s** some really good, but very difficult questions you ask - but i will try to answer them as best i can. Keep in mind, that a lot of how to build your characters defense is still a lot up for debate. But i will state my thoughts on the topic.
Q2: Is the blocking itself the main defense?
Blocking is right now a really good defense, due to how high the block protection is on the best shield in the game. 1600 block protection + t5 suffix for 600 block protection. There is no other way in the game, where you can get such a high defensive bonus, with such a low investment. This make blocking really good for low lever characters, that have not been invested a lot of time in - but it will begin to flat out in endgame (i will explain later).
Blocking however suffer from one crucial thing, and that is, you can’t maintain 100% block chance. If i remember correctly you can reach 93% block chance on a paladin (and on other classes even lower). This is really good, but it causes some issues. On top of that, blocking, is only reducing hit damage and not DoT’s. On sentinels and in general the more tankier classes, DoT’s tends to be their weakness. 1) Because glancing blow Does not affect DoT 2) Because tanky classes tend to have lower health gain / leech / regen - so they are even more vulnerable to DoT’s.
Depending on what purpose you have for the character, a 7% chance for not blocking can be fatal - especially for high level characters. If you only use blocking as main defense, you would most likely experience one shots now and then. Which in most scenarios forces characters to use a second layer defense.
Here you have the options, Health, dodge, Ward or increase your protections further.
Q1: If I have ‘good’ blocking % for my level, do I still need protections (elemental, void, poison, necrotic)?
This highly depend on what class and materie you are playing. If you stack protection, it will be additive with your block protection, each time you block - so it would still be really good on certain classes. I used it on my paladin as a main defense, with blocking as an secondary defense. If you however tend to use block as an main defense, you problably would like to focus a lot on poison and necrotic, since they tend, to cause the most problems with damage over time.
Stacking Ward trough gain on hit idols, or Holy aura with blocking, is an extremely good way to mitigate the drawback from Block protection, since there is no cap for Ward and it will increase you eHP by a lot. The ward will then be “saved” for the bad times.
If you are curious to learn a bit more about the ward stacking on protections and blocking, you can read one of my older posts: Have ward finaly found its way to protections build? - HolyCoffee
Health and dodge i think, would problably give you the highest effective health pool. But the problem with dodge, is again the consistency. And consistency, seems to be a big factor for ARPG’s.
Last points about blocking
So if you are playing a sentinel, you can also get some really good idols for block protection that gives 500 (best rolled). You can have 4x of them, which will be total of 2000. All this block protections is so SO good, for how low investment you are making.
But when you start to minmax your character, block protection is loosing its value. This is because, of how few ways there are in the game, that actually can %increase block protection. As far i can remember, ring of shields, is the only way to actually increase the block protection, although its with 50% (so its really good). So maybe on a fully completely insanely minmaxed character, maybe block protection is not the right way to go, because all the %increase amour and protections and %increase elemental protection does not scale with block protection. This means, that if you have a two hand, with t5 affix 600 elemental protection, you need to have around 150% increase protections, to make it give the same, as the base of the shield. Although 150% increased protections is a lot, it is definently achieavable to get close to that amount. and at that point, you need to ask yourself, if the damage you sacrifice from swapping to shield, uplifting the defense. It problably will not be. But due to how good health gain on block is, and the reduction to dmg on block - it’s an really close call. If arena was a lot “shorter”, the would definently be the way to go. But going trough 300+ wawes, and hoping for all one shots getting blocked is a long shot.
A last note i will add is, that rallying block in forge guard, is one of the best nodes for block users, since it will increase your flat protection, so it goes well with, a pure protection build. And health gain on block (and ward), is also really good, and was the main reason i did use block on my paladin
Sorry the long ramble, and i am kinda thinking i am missing something. As you can hear, i don’t have a clear answer to block, it depends on a lot of stuff. And as you also problably can see, i am still conflicted rather a not it is in “general” better go block or not go block hahah, which is the reason the tone is changing so much haha.