xxxThaoSxxx

I believe their debuff can be countered by any type of coressponding buff, and it doesn't have to come from your character, it can be casted by your party member, allowing you to go full berserker (without multiclass) for a much stonger character overall.

Which basically means it's only useful for tanks, or should DPS melee characters focus on it as well? I think its useful if you can raise it to at least 15-16, play as class with decent deflection progression AND have access to various other deflection increasing abilities. I am not sure at this moment if there are DPS characters who can satisfy those conditions while also having stats left to be actually good at dps.

As far as I understand, it is a very powerful defensive stat if maxed, but not very useful if not maxed. Basically you need it to provide you maximum amount of deflection so that it would be higher than accuracy of your enemies.

There is a nerdcommando video: https://youtu.be/1HrfPljMfGY?t=18m29s on classes, here's the part he talks about Chanters (Beckoner is the first one he discusses, so you don't need to struggle through his video for too long).

Beckoner is specialized in summoning branch of Chanter skills. If you go multiclass you will lose the ability to summon the most powerfull creatures from Chanter school. It seems to me that there is no good multiclass which will compensate the loss of the most powerful summons. And since you rely on your summon's strength, not your own - you can gimp you own Strength stat and focus on maximizing Resolve (deflection) and other tank stats, providing you with survivability you need.