Considering that 100%ing the game is an optional thing, I don't think it's a "tacked" on feature as you would like to call it. It promotes exploration of the level, which I do like. I've always liked collecting them in Super Mario 64 DS, though it's a ----ing pain if you die when you fall off a pit.
Really these missions are not as stupid and tedious as those damn 100 purple coins in Galaxy games, mainly because collecting Blue Coins and Red Coins adds to the count and Galaxy's levels aren't really built to be the exploration type anyway, so it sucks badly.
If you want a tacked on feature, look no further than Galaxy 2's *bleeping* Green Stars