I'm not sure what all you expect to realistically find in a single embark area. I could see one or two large veins of a single metal and then some other tiny trace metals scattered around, but most places SHOULD only have one or two types.
And his point is this is boring.
I agree, that my ideal would be having (most?) sites having 1-3 "common" metals that are in large quantity, but then any number (as in, 0+) of "rare" metals scattered throughout that would not register on the site finder, so there'd be a point to exploration, because you just might run across a platinum vein the sitefinder failed to pick up.
I liked stumbling across things like platinum and aluminium before. Now it's true that a quick survey of an area (usually not more complex than digging to the third cavern, doable in less than a season) will give you a full inventory of everything you can expect to find on your site. However realistic that is, it's boring. And ultimately this is a game, not a simulator.
This problem will probably be addressed by the world gen options Toady mentioned, though, if only by a return to .18 material density, although personally I would prefer the option of keeping the game as-is but with occasional rare, unshown veins.
Maybe even add in a few more "mythic" metals like mythril or what have you to be these hidden metals.