Under down tube cage- which one, if at all.

NBD (road) is approaching, well it was till I put it back 2 months for a new colour...

Anyway its got a set of under down tube bosses, which i thought would be useful for a cage with a strap- for shedding a jacket or carrying an extra bottle. I was thinking a set of King Cage's with their Mini Thing one underneath. 

If I'm riding in variable weather (hence too many layers) isn't anything strapped under the frame just going to get covered in nasty grimey road water or worse! Similarly when its empty wont it just be a faff to keep clean. I will ride it all year with the addition of full mudguards in winter.

How useful is this extra cage- is it really just for proper touring?