Concentration for Martials

[Witty Username]

I mean, a game element can be both a resource and a constraint.

To expand on this, resources are constrains. They are limits on the amount and context a character can do something.

Being able to spend a resource is not an advantage, things like spell slots, concentration, ki, prof uses per long rest, etc. Are fundamentally not helpful without an ability to use them.

If you have two characters, one has a +2 to attack rolls, and one has +2 to one attack roll if they spend a point. The one that does not require spending points is simply superior unless anothers affect is in play.

For this, use concentration if it looks like it will create an interesting play state or limit a powerful ability. But it is not a slot, not needing concentration is an advantage, not a limitation.

--
Spellcasters don't normally poach martial abilities by multiclassing, the tend to duplicate the effect with spells or invest a subclass as possible.
And as a hot take, they tend to be the weaker options or have unfavorable play in the martial space.