Only blocks built as armor (paid for at 2:1) appear to reduce the actual effect vs the action's roll.
I think there may be a misunderstanding. If not, I'm happy to be corrected.
Evocation Blocks are supposed to reduce the shifts of any action they are intended to block, AND if a given action roll produces shifts in excess of the Evocation Block, that Evocation Block goes down - this is a special limitation of Evocation Blocks. It still reduces the shifts of the action which broke it. The motivation to cast an Evocation Block as Armor (2:1) is that, as Armor, the Evocation Block does *not* go down when a given attack exceeds it. Per YS 252: "
Block: Shielding effects are another very common application of evocation; the next best thing to causing damage to others is avoiding it yourself. In game terms, this is a block action. Shifts of power on the spell can be allocated as follows: 1 shift of power adds 1 to the block strength of the block action. Three shifts of power create a block strength of Good (+3).
Any attack that bypasses the block cancels it out."
Mundane Blocks not only reduce the shifts of any action they are intended to block, but when a given action roll produces shifts in excess of that Block, the Block does not go down. It still reduces the shifts of the action which exceeded it, and it remains in place until the character decides to do another Action. The Block has been nullified for the character who exceeded it, but not anyone else.
Edit: YS 210:
"To perform a block, declare what specific type of action the block is intended to prevent and roll an appropriate skill. The total of that roll is called the block strength. During the exchange, any time a character wants to perform the action that’s covered by the block, he must roll against the block and meet or exceed the block strength to be able to perform that action. If he fails, he cannot perform the action in question. If he meets or exceeds the block strength, the action resolves normally, with benefits for extra shifts if the roll beats the block strength by a wide margin."
A visual example of this is when a Mercenary is laying down cover fire (rolling Guns) to make a Block:Movement action to keep a group of intruders from reaching a critical locale. When one intruder exceeds the Guns Block with an Athletics roll, that intruder successfully makes the desired Move action (minus the shifts it took to meet the Block), but the Guns-based Block:Movement is still applying to the intruder's companions. This is an expansion of the example on YS 210: "For example, if your character is in a gunfight against a group of goons, you could say, “I want to pepper the exit door with gunfire and make sure no one leaves.” That is a block against multiple characters, so it can only prevent one type of action, which in context is any move action—no one can leave without running up against the block."
So, per this understanding of the rules, a Harrying action is redundant.
This would seem to make Mundane Blocks more effective than Evocation Blocks, and in some ways they can be, but Mundane Blocks still need to have some plausible narrative basis, while Evocation Blocks are narratively self-justifying. Per YS 211: "Keep in mind that there are some blocks that just won’t work in some situations. (Trying the “keep them pinned down with gunfire” trick on a loup-garou isn’t going to really help you much, given that they’re immune to bullets.)"