An excellent question, and one I asked myself when I first started using the Metalizer paints. Testors says that their Metalizer thinner #1419 is specialy formulated to provide more "bite" to the paint, and to ensure the paint cures completely without remaining tacky. However, since the Metalizer paints are ready to use straight from the bottle without thinning, the only time you'd need to thin them before use is for a "wash" or a semi-transparent application (say, applying a thin coat of exhaust over stainless steel). I keep a jar on hand for those uses, and it's lasted me forever.
As for cleaning the airbrush after using Metalizers, I use plain old laquer thinner from my inexpensive gallon can, and it works just fine.