You may as well spec one prot. He will be able to take damage better than a ret and put less stress on heals.
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This. To be more precise, bouncing aggro is tricky and prone to error. Furthermore, one good prot tank can absorb more effective damage than 4 rets combined. Eventually, the content you're doing will outstrip your healer's ability to keep up. That threshold is a LOT higher with a prot tank than with rets.