CPU's a big help if you're compiling a lot. Then again most projects are small enough where it doesn't matter. But my MBP's a necessity when it comes to compiling Firefox.
Even compilation is mostly IO bound. I'd be interested in an off-the-shelf benchmark on compiling Firefox, MB vs. MBP. I haven't seen one. Regardless, you're paying a lot for what may be a nominal difference; you could spend that money better.
