I've sworn that I'll never buy another bike without discs. Certainly for riding in traffic I consider them mandatory.

Some real-world disadvantages:
Harder to get a rack and pannier to fit -- but still doable, I've got two bikes set up that way.

Spoke repair is harder if you break a left-side spoke. You have to take the disc off, whereas on a non-disc bike you can sometimes do that repair with the wheel on the bike. One of my bikes has straight-pull spokes and does not have this disadvantage.

Not an inherent disadvantage, but a flaw of the Avid mechanicals my bikes have: The rear brake is the same as the front brake, with a different mounting bracket. The return spring on the rear needs to be stronger to compensate for the longer cable on the rear and the attendant friction. The rear has a tendency to stick when applied. Not really a problem for me as I haven't used the rear brake in years.

Sure, weight and cost are issues, but with any bike part you can buy things that cost less and don't work so well.