Open Bath: oil used for damping is also used to lube the sliding elements of the fork. Dirt that gets past the seals will not only ruin the stanchions and bushings, but will damage your damper too. Regular oil changes = good.
FIT: Oil in the damper is seperate from the semi-bath oil and does not intermix. It's considered superior because the oil to lube the sliding elements can be an optimal weight for its purpose, and the same for the damper. Also, while the semi-bath oil might get filthy during regular use, the damper oil remains relatively clean an can do its job well for a longer period of time.