This spell isn't useless, but I would agree that it is less useful. In the shadow example above, you're correct; a magic weapon spell would do the trick. However you can't refute the fact that Magic Missile would have SOME effect there, so it isn't useless, just less useful.
Not every spell is a win button. Heck; a lot of them are not. However one thing I (along with Dr Deth apparently) get frustrated with is folks saying less useful = useless. It was demonstrated upthread that MM is more useful than Burning Hands long term if you factor in saves, energy resistance and range. However this does not obsolete Burning Hands by any means as the spell has its own merits that it is a cone shape and that it ignites flammables in the area of effect. Just as MM is situational, I could see Burning Hands' extras coming in handy in certain situations; say a bar fight. One spell and suddenly there's a 15' cone shape of the bar, liquor, and folks' alcohol-stained clothing on fire, reducing the action economy of anyone in that vicinity and potentially causing the place to empty.
All I'm saying is that no option in the game is fully useless. It wouldn't be added if it didn't add SOMETHING to the potential success of the PCs. However I agree that some choices are superior to others in terms of their value add. Magic Missile, as it stands without any special build or mechanics added, is one of those lesser options. In a pinch however it can be utilized to contribute reliably, albeit miniscule, amounts of damage toward ending a threat to the party.
Just remember: part of the formula for DPR is chance to hit. MM hits, every time. Not much actual damage, but it's a 100% chance of success. That's not too shabby.