While I understand the logic of fearsomeness in Skeletons, I have 1 gamey and 2 non-gamey arguments against it.
Gamey argument - If you give the Skeleton Horde the fearsome attribute, you'd also have to give the Skele Spears, Cavalry, and Bowmen(!) the fearsome attribute. So now you have 8 out of the 12 Undead units being fearsome. Might as well make the entire faction fearsome at that point.
Non-gamey argument 1 - Your soldiers already know that they are facing an army of the dead as soon as your general tells them that they are going to march out there and face the forces of the Necromancer. It's a given. Even if these soldiers have never faced Undead opponents before, I'd bet that their initial reaction would be something along the lines of "Fighting Skelis is freaky, but I'm sure darned happy that we're not fighting that Giant thing way over there that's been chucking boulders at our Spearmen. And it's a heckuva lot better than facing off against that over sized blob of guts, gore, heads, and limbs about 50 paces down yonder."
Non-gamey argument 2 - I can think of quite a few units off the top of my head without the fearsome attribute that would probably be more frightening for your typical solder to face than a unit of Skeletons. Tell me if you'd agree that the guys listed below would be more likely to cause you to poop yourself if you saw them charging right at you on a battlefield:
Lizardman Tyrants (either variety) - 7 foot tall, 300 pound armored reptiles
Lizardman Raptor Pack - Ever see Jurassic Park?
Any Heavy Cavalry - Seriously, this is almost the equivalent of being a foot soldier in WWI and seeing a tank bearing down on you.
Centaurs - See above, but they are also throwing stuff at you as they charge! Plus you must confess that Centaurs are pretty freaky by nature.
Ravenwood Bear Pack - Seriously, are skeletons scarier than this? I thought not.
Any kind of Elder Blade Wielding High Elven Unit - OK, maybe not scarier than Skeletons. But pretty close I'd bet.
Edit: Besides, the 3 men of Willowdale where able to survive the dungeons of the Necromancer
