count_all_results() - is it optimized?

2. The DB classes are designed for portability....

I'm wondering how often the need for this portability happens. I've only had one instance where portability might have been useful, but the app was a complete rewrite anyway. In a lot of apps I've worked on, I've had to write such complex queries that I couldn't use the query builder, and so the portability is broken. Is it common for people to one day simply decide that they're going to switch their database and the portability is actually a benefit?

Well, it requires discipline to maintain your codebase in a portable state, and to be honest it's not worth it just for a potential switch. The actual benefits come if you're developing an app that you would distribute for others to install (like a CMS, for example), and they then have the choice of which database platform to use.