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The main advantage would be the integration wih users, groups, permissions, etc.
You could wrap the existing in an I-Frame from Xoops, or run it stand alone. This would not be intergrated as staed above though.
As for sharing a DB. You would have to hack the XOOPS module to use the stand alone product's tables. This is doable, but I think the same amount of effort could produce some SQL conversion scripts that would give you the integration you most likely want.