Because having 2 processors is not the same as having 1 processor that's twice as fast.

Have you ever picked up a pencil and written a word down on paper? Try doing it with a friend. You write one letter, he/she writes the next. After about 3 letters you'll realize that it would be a lot easier if you wrote it yourself.

CPUs are exactly the same as human brains. They need to have their own job. With 2 procs, you'll get better performance, if and only iff there are jobs that can be done in parallel. As far as I know (knowledge gained in these forums) Vegas will use 1 proc for the intra frame work (filters/effects) and the other proc for inter frame work (compression).

I have 2 procs. The only reason I have them is because I could afford them. If I was on a budget, I'd get the fastest single proc system I could get. Because that's where the bang for the buck lies. I like to have 2 processors on my machine because it makes my clients think I'm doing something they can't do. Also, when I'm rendering a project, my machine is virtually unnafected. I can fire up another Vegas and keep ripping.

Also, I use after effects alot. If you use 3D space and tons of filters, it'll start using both processors like there's no tomorow.