The spare man in central midfield is the whole point — you free the pivot to receive on the half-turn and suddenly the block has to step, and once it steps there's space in behind.
Tucking a full-back into midfield gives you a spare man in build-up and an extra body to overload the half-spaces. Is it genuinely unlocking deep blocks, or just moving the problem and leaving you exposed on the counter?
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The spare man in central midfield is the whole point — you free the pivot to receive on the half-turn and suddenly the block has to step, and once it steps there's space in behind.
Great until you lose the ball. Now your full-back is 30 yards from his flank and the winger has a highway. Against a good transition side it's suicide.
It's not about the full-back, it's about the rest defence it enables. Two pivots + inverted FB = you can commit five to the attack and still be solid on the counter.