It seems like Concentration is frequently knocked off anyways even with it; and it only works at the start of the combat? Am I missing something here?

I can make all the difference. For example:

You want to cast Slicken in order to interrupt a whole bunch of enemies. But they shoot as you with some sort of interrupt. With Combat Focus you can still finish the Slicken and now they get interrupted every 3 secs. Without Combat Focus you will get interrupted, your spell use is gone, the enemies shoot at you again and angain and you will be sad and dead.

It makes zero difference if several enemies fire some sorts of interrupts at you. BUT: you can stack Concentration. So if you get some other sources like from an item and another ability you will three layers of Concentration. 

I had a solo SC Furyshaper centered around Barbaric Retaliation and Blood Ward and Combat Focus + some other sources of Concentration made a huge difference. Without Concentration I could not for the life of me summon a Ward because I got interrupted all the time and died quickly. With some layers of Concentration I could get the Fear Ward out and enemies were screwed. 

You don't need concentration if you are wearing Rekvu's Fractured Casque with an injury. 

49 minutes ago, Boeroer said:

I can make all the difference. For example:

You want to cast Slicken in order to interrupt a whole bunch of enemies. But they shoot as you with some sort of interrupt. With Combat Focus you can still finish the Slicken and now they get interrupted every 3 secs. Without Combat Focus you will get interrupted, your spell use is gone, the enemies shoot at you again and angain and you will be sad and dead.

It makes zero difference if several enemies fire some sorts of interrupts at you. BUT: you can stack Concentration. So if you get some other sources like from an item and another ability you will three layers of Concentration. 

I had a solo SC Furyshaper centered around Barbaric Retaliation and Blood Ward and Combat Focus + some other sources of Concentration made a huge difference. Without Concentration I could not for the life of me summon a Ward because I got interrupted all the time and died quickly. With some layers of Concentration I could get the Fear Ward out and enemies were screwed. 

You don't need concentration if you are wearing Rekvu's Fractured Casque with an injury. 

Got it - it will stay on my casters. And thanks as usual!

it's lower on my priority list of passives, but nothing is more annoying to me than my buffers trying to cast their initial round of buffs and having an important one get interrupted by an enemy rogue doing an unnoticed shot with one of their blinding attacks. it especially stinks earlier on where i am more likely to be using a buff spell that i only get once/encounter.

 

though whiteleaf is an extremely easy source of start-of-combat concentration as well.

almost all skills can receive the same effect via equipment , food, or buffs. that being said, I think having concentration at the start of a fight is very beneficial as mentioned above. I usually take this skill