I'm a bit on the fence about this, but the old "he's not a max guy" argument in meaningless. There is no special meaning to "max" - just a salary and a contract once it's signed. Some guys can get $21M, while others can make $30M. Some will be signed this offseason under one cap, some next summer under another. Some are 30 years old, some are 24.
When Barnes is making $22M under a $108M cap, it will be equivalent to $14M under the current cap. Is he not worth $14M during his prime years? Is $22M just another "max" deal when some other players are eligible to make $35M next summer?
As with all these things, it matters what other moves we make. The money is just 'ok' depending, IMO. He projects about the same as Crowder in terms of value... 15/6 ish with good defense and ability to play both forward spots. The money has to be spent and he's a useful piece.