I decoct quite frequently, maybe as often as every other beer, because I find it a convenient way to do step mashes to control fermentability for some beer styles. I don't expect and I don't really see any obvious flavor differences from the standard recipe, so I wouldn't add any melanoidin malt unless you think the recipe would benefit from melanoidin malt flavor.
To the other point: not that decoction won't make your beer clearer, but both my decocted and non-decocted beers are crystal clear. I think water chemistry is the primary factor instrumental in achieving clear beer, not boiling the grain.