I think we've got Dumbledore to thank for this one, honestly.

CanonVoldemort went to every effort to kill Dumbledore, but it seems clear that RationalistVoldemort could have murdered him at leisure. He keeps Dumbledore around as a sort of Batman to his Joker. It puts the story where he taught him not to respond to hostage taking in perspective.

Despite his disdain, Voldemort has to recognize that Dumbledore's madness is occasionally an advantage. Since he elects not to kill Dumbledore, he has to compensate for it. I'd argue that he actually enjoys it on some level.

Thus, while all logic has Voldemort dead long ago (for all the reasons Harry/Quirrel discuss) Dumbledore leaps to the conclusion that Voldemort is alive and behind everything. That is, he leaps to the correct conclusion, for no reason.

Similarly, given a belief in the existence of Voldemort somewhere in the world, any other wizard would be searching diligently and inventively. Dumbledore, by contrast, sets up a trap that couldn't be more obvious if it was a box propped up by a stick. Quirrel openly mocks it earlier in the tale.

Its a trap that should never work, and yet, here we are, with Lord Volodemort entering the trap.

Harry is, to Voldemort's mind, mostly just a subset of his own abilities. Not as smart, shackled by stupid teachings, not as magically powerful. If the creator of the defenses was some kind of RivalVoldemort there would be no need whatsoever for him to bring the potentially traitorous Harry, but the defenses were designed by Dumbledore, and will be just as stupidsmart as you'd expect.

The traps set up by faculty are more or less just foreplay. They are set up by the Snapes of the world, just weaker and dumber versions of Voldemort. But Dumbledore's might be a whole other kettle of fish.

Perhaps, in order to reach the mirror you must pass a Dementor? Its a stretch to imagine him having a dementor in the school, but maybe its trapped somehow so it can't kill students?

Maybe you have to persuade a phoenix to bring the mirror? Or casting the spell to get to it requires your life energy so that if you don't have a phoenix giving you strength you'll die in the process.

Perhaps its as simple as the magical equivalent of a rock that only Dumbledore is strong enough to life. Or it is just keyed to only open for Harry Potter, because Dumbledore irrationally believes that Harry will be the next Great Hero.

The edges of Voldemort's abilities, those minor tricks and habits that Harry has that Voldemort hasn't deigned to acquire yet, might be the key to Dumbledore's final barrier. Its worth bringing Harry along for that.

Beyond that, I think there is a bit of Steerpike syndrome here. Voldemort is probably, on some level, lonely, and Harry/Tom is useful as a sop to this.