Probably a matter of preference.
-Less line on the hoist as the tack is already pulled out.
-You can do it from the wire.
-Easier hoist as the spin is partially pulled from the bag already.
-I'm completely retarded and can not figure out how to rig the block running up the mast when the mast is laying on the boat instead of up.

Down sides
-You have to release the tack on douses

I'm sure there's other negatives, but that's all I can really think of.