verification of some sort should be a requirement, but in some cases domain verification is harder to do than a single email address verification. I don't always have control over the DNS (which is always how I've seen domain verification done), and getting access to it, or having someone change it for me, might take days or weeks (or get done wrong, or whatnot). I'm dealing with a couple of clients right now where any change request like that takes 2 weeks - that's just standard policy. But... we can verify "foo@company.com" as a single address immediately.
This isn't to say domain verification is a bad thing, but depending on how it's done, it's not always the first thing you can tick off on a project.
I was saying that sometimes you can't update any of that quickly, but you can - via amazon ses, for example - validate a single address. Doing that will allow you to get past at least part of a project's set up and keep moving until you can get dns issues updated. it's not a 'never do it', but 'that can come a bit later without being a roadblock' issue, imo.
But email is all externalities. Is the convenience of you being able to send email more important than the anti-spam management of "yes, we actually own this thing"? (icebraining's points regarding DKIM and SPF are more practical and, IMO, accurate, but the collective problem is more interesting to me.)
This isn't to say domain verification is a bad thing, but depending on how it's done, it's not always the first thing you can tick off on a project.