This is an excellent depth move to solidify the roster, similar to moves Pete and John are lauded for making.
Ruggiano is a good platoon bat and a reasonable defender in corner outfield. In a pinch he can man center, though he is not particularly good there (then, neither was Saunders). He doesn't have significant upside, but is cheap, here fore two years, and fills a desperately-needed 4th outfielder role.
We still don't have a 3rd outfielder, so obviously the roster is still incomplete in that regard, but the Mariners needed a backup outfielder, and they gave up a relief pitching prospect for it, who hasn't yet even advanced past AA. Relievers being as volatile and overvalued as they are, they essentially gave up nothing and added a valuable piece to their big league roster.
In essence, Mariners parted with nothing, gained something. Ruggiano shouldn't be an everyday outfielder, but if he were platooned with an Aoki?