If you're willing to just solder wires down, there are some potential cost savings in doing it this way over anything other than a cheap-o digital controller that you happen to find in the clearance bin at a gamestop (which hopefully actually works), but they're minimal.
The real reason to do this is so that you can map things in ways which aren't possible with a standard gamepad hack, like hooking up a 49-way and having it emulate the left analog stick.
I have digital control emulation working, and am working on original dualshock (DS2 is harder) w/ full analog support.