Really everyone should have some sort of “framework” for CSS right? After repeating similar work countless times you eventually get a “framework” going where you organize your work in a certain way and create a base to work from. If we were not coding with CodeIgniter then we would have our own basic PHP framework to build from. You borrow your own code from other projects and re-use.
That is what BluePrint and YUI try to do. I am using both of the above mentioned frameworks in a personal project (one for the admin and another for the public facing areas) to get a chance to evaluate both of them. I’m not sure I can say one is better than the other right now. BluePrint is limited in some ways (unable to create fluid layouts) and YUI has a bigger name behind it. ExtJS is actually bundled with YUI so you might check that out.
I think it’s good to have one or a few css base files but using a framework seems a bit overkill to me.
I think you are just getting caught in the semantics. A few CSS base files is essentially all that BluePrint and YUI are. Each have extra items that you can add or remove. You can also combine these items and “compress” them to decrease the size of the files. There is a bit extra fluff but I think that is a small trade-off.
If nothing else, I have found these frameworks great for throwing together a fast layout to work from in my projects. When I am trying to code I hate getting bogged down in CSS (which I am very rusty in.)