Everything I've read says Spears is better fit for a 3-4 scheme at the nose. Why would it matter as a DT? Wouldn't he work just as well with another DT in a 4-3 to collapse the pocket?

I dunno either. It doesn't make sense. Fools like John Clayton and others in the media are fascinated with 3-4 defenses and like to typecast players. Spears is athletic enough to play DE or DT in the NFL and be a 3 down performer, effective vs pass and run, in any defense.

The 3-4 defense works partly because teams save money running it. They usually prefer large DEs, sometimes moving DT to end and a hog at NT. Their job is to just hold the point of attack and almost nothing else. They rarely make tackles. They save money not taking the pure pass rushing DEs that command big money. LBs generally come cheap but must be athletic and in 3-4s there is less responsibility and more blitzing. Allows them to use younger players. It does require a good defensive coordinator with creative and intelligent play calling.