The only phone that truly gets untouched virgin updates would be the Nexus One, and even then the drivers are added by HTC.
The Moto Droid I guess would be the next one to also, but its been so long since I used a purely stock version that I remember if Moto adds apps at Verizon's request.
All carriers request what features/bloatware to install/not install. They send said requests to OEM, which turns around and incorporates the requests, and adds their custom branding/widgets/etc to send back to the carrier for testing. The carrier then has its own internal testing to verify requests and make sure no new prevalent bugs pop up. No one at Verizon (or any carrier really) actually does any coding of any kind.
What the OEM's receive from Google may be what you consider a "pure ROM" with "hooks" built in so that the various OEM's can add their custom homescreen, bloatware, brand specific widgets, etc. But, with the source being released most likely what happens is that this job now falls upon the OEM's themselves to write in their own "hooks", as Google just pumps out strictly OS generic builds into the ASOP.
In which case, either Sammy or Moto or whoever pulls that and fixes whatever issues, or they have internal builds going (which is more likely) that they tinker with to fix issues.
I believe Moto does this as evidenced by the lettering of the builds. Seems like they go through builds nightly when they are working on an update for a phone. I wouldn't be surprised if Sammy was the same way.
Sorry to make this post so long. Hope this helps.
Current: Samsung Galaxy S3 (SCH-i535)
ROM: Jellybeans Build 4
Current (but missing): Samsung Galaxy Nexus (Toro)
ROM: BAMF Paradigm 2.3.5
Radios: i515.10 V.FC04/ i515.FC05