Why would you push from your Build Server?Why wouldn't you?
Seriously. The role of what we call "build servers" has been dramatically expanding. When I started in this business, nightly automated builds were the state-of-the art. Then we went to automated build systems that would build whenever new code was checked in. Then the build systems would run tests as part of the build. Then the build systems would spit out reports of automated test coverage so you could know if you were missing something you thought you had covered.
Also, I wouldn't ever deploy production code that wasn't built by the build system. So I'm having trouble with the "nothing to do with deployment" argument. It has everything to do with deployment.
I don't consider my TeamCity install a "build system" I consider "building the code" a subset of the overall Continuous Deployment system that's powered by TeamCity (which is amazing, cross-platform, and free for small teams)
The end point of all of this is that instead of releasing every few months or weeks, I can release every few hours or minutes. Basically, every single change that isn't demonstrably broken gets released without us even thinking about it. It's transformative.
What's the safest change you can make to a stable production system? The smallest change possible.
When do your customers want to get their hands on a bug fix or a new feature? Right fucking now.
If something does go wrong, how many changesets do you want to go through to try to find the problem? As few as possible. One is ideal.
How many new features and fixes do you want to have to roll back if something goes wrong? As few as possible. One is ideal.
Some specific questions:
Rollbacks are done by just re-pointing IIS manually. I've only had to do it once. Alternatively, I could have backed out the change and pushed that through the continuous deployment system.
As far as DB Schema changes go... Again, the safest change to make is the smallest change possible. Generally, I make DB changes that are backwards compatible (adding new tables, adding columns to existing tables) so if I need to roll back the code, it's just good. If you're tightly coupled to a lot of logic in stored procedures (I'm not) you're going to need to make their changes backwards-compatible (generally by using default parameters) or embed a version number in the procedure name (e.g. "InsertEntityV2", etc.) to let the two versions of the proc live side-by-side.
If I need to make some sort of breaking schema change, I would have to do a scheduled downtime and handle that particular deployment mostly manually. As far as I can tell, there's no real easy way around that problem (IMVU also does their schema-breaking changes outside of their continuous deployment scenario).