It depends on whether or not I'd plan on adding it to something bigger. One good thing about frameworks is that you can use the same codebase.
If the framework gets upgraded, everything gets upgraded by replacing one directory and all the code you wrote does not have to be touched (unless you know about deprecated items well in advance).
I use Codeigniter for a lot of stuff, and basically just keep adding my own libraries to it as I come across things I need that I don't already have.
Just make sure you use some type of system for documentation like phpdoc, or you'll end up writing stuff you forgot you already wrote 6 months ago.
I wish I knew complex PHP, but I do have a solid custom HTML/CSS/PHP framework that I've put together from the best pieces of some of the other frameworks out there (Tripoli, 960.gs, Typogridphy, Baseline, Eric Myers reset, etc.) that I use as the starting point for ALL my websites now. Saves a lot of time.
Yeah, this.
Sometimes. Just be practical about it. Pre-optimization and over-architecting will just get in the way of what you're really trying to do. The keyword is "big" here, and the larger a project gets the more you need to have consistent conventions ... which frameworks can provide.
Me 3. I have sort of a rough framework that I use for this, basically a set of folders called "blank site" (/css, /images, /js and a blank index.html). In CSS I have the 960 framework stuff and a global reset loosely based on the Eric Myers one. In JS I have minified jQuery, which sometimes I end up yanking out. Whenever I make a new site, I just copy this structure and rename the folder to whatever the project is. Saves me some time, but not a ton. Curious to see what you're doing though.
Just make sure you use some type of system for documentation like phpdoc, or you'll end up writing stuff you forgot you already wrote 6 months ago.