As always “It depends”. 
If it’s a proprietary application which already has a deb built, and the vendor doesn’t have the resources to look into snapping their application right now, then ingesting the deb and making a snap from it is certainly an option. However the ideal scenario is that the yaml lives in the upstream source tree, which is hooked up to their ci system whether that’s travis, circle-ci or some other thing, and spits out a snap. That snap can then be pushed directly to the store edge channel for adventurous users.
If you’re talking about debs included in a build of something else, we often use stage-packages to pull in things from the archive, but ideally we should be building from source each time. stage-packages is really a convenience, as it’s way easier to just specify a stage (or indeed build-package) than work out the necessary parts one by one, with all their build intricacies.
Personally I’d like to see us move all our things to source-based builds rather than stage-packages and build-packages, but building all of the world every time is a massive consumption of resources, so a balance needs to be struck.