For me it's mainly as you said on another comment... The overhead of managing and switching between branches is just too big a price, for no gain that I can see.

I have a hard enough time remembering the state of a single trunk, let alone if I had several branches on the go at a time. I'd fail spectacularly at remembering which branch has what on it...

It's easier to just do:

  * Never break the build / functionality of code (Good idea anyway)
  * Architect your stuff well with minimal dependencies so you can
    can swap in/out modules/components easily.

Personally, I feel branching+merging is as much use/fun as filling out TPS reports.