I like having the computer be really snappy all the time regardless of what I am doing, that is why I like dual-cpu systems. Had the BP6 for a while and now have the Tyan Tiger with dual 1900+ and a healthy 1.5GB of PC2100. Nice and snappy indeed.
The way I see it, you sit down to use a computer that costs maybe $2,000 if you add in all the components and accessories (printers, monitor, digital cameras, software, etc... you know all that stuff), so you spend an extra $200 bucks (extra $100 for MOBO and extra $100 for a 2nd processor). Now you spent $2200 (whoopee, an extra 10%, just don't go out to dinner or order pizza five or six times and it was essentially free) and your computer is really snappy regardless of whether you use multithreaded apps or not. If you already spend so much money anyways, I figure it's worth it to spend and extra %10 and gets a Rolls rather than a K-car.
My 2 cents
-Phil