Quote Originally Posted by JLandan View Post
Whatever is used for the "concentration slot", remember, Barbarians can't do it while Raging, Druids can't start doing it while in Wildshape, no gish can do it while holding a smite spell, and rangers can't do it while using Favored Foe.

My opinion is that if this feature is open to any class, it must be open to use for casters while not otherwise concentrating. If it is only open to martial classes, how does it work with multiclass into casters?

Some things to consider.
Indeed, if the "4th slot" conflicts with normal concentration, then it becomes more of a tradeoff for gishes and casters than it is for pure martials. (I think some folks would consider that a feature rather than a bug.)

Quote Originally Posted by stoutstien View Post
The fact the concentration is looked at like a slot to be filled means it probably not something you want to build off of the begin with.
I think it's only natural to think that way honestly. For many, it's fun to feel like they've maximized every "slot" on their characters - a productive use for every action/BA/reaction, every attunement slot filled, concentrating on something etc.

For example, on archery builds I'm constantly trying to figure out good uses for my reaction since I'm probably not going to be using OA or Ready very often.