I'd take a Legion of soldiers led by a dozen low level Paladins over an extra plus to my weapon any day. Even against level 30's, the soldiers will still provide more benefit than an extra + would on a weapon. I agree they wouldn't be able to do much harm to a level appropriate opponent, however the bonuses they give to allies due to flanking, aid another, blocking the enemy, etc, far outweighs a measly +1.
For the first round. Then they are incinerated by an AE attack (again, this works for any version of D&D...the main question for me, in 4e, is are the rank-and-file rabble Minions or Soldiers? If the latter, some might survive the first round of combat. Unfortunately, the 4e answer seems to be "Whatever they need to be for the sake of the PCs", which makes worldbuilding exercises difficult...).
The extreme abilities of high level characters in any version of D&D is one of the conceits which drive the implied setting. They're why someone pays 100K gold for a golem to guard their tomb instead of hiring a huge army to do it -- because a high level rogue can slip past that army, a high level wizard can incinerate them, or a high level fighter can just kill them all.
Are there scenarios where an army is more useful than a single high level character? Of course. But there's also cases where investing a million gold in an army and investing a million gold in outfitting one hero is not a simple no-brainer. An army of low-level types against an epic threat -- elder dragon, demon prince, whatever -- is almost certain to simply die, and that's a million gold you COULD have been spending on teleport circles to get the hell out of the way.

(It's interesting to see how the rules changes alter this. In 4e, the army is a somewhat better investment because each flunky has a 1-in-20 chance of doing damage and doesn't need to overcome DR; in 3x, it was literally impossible, even on a critical, for a 1st level warrior with a longsword to do ANY damage to some high level foes. Branching out, in Rolemaster, an army is very useful, because if you throw enough mooks at a problem, one of them will roll an open-ended result and get an 'E' critical and that'll be all she wrote...Of course, in classic Runequest, 5% of your army will slice off their own leg in each round of combat.
