well of course if you got the cash go for it. But it is a viable option to run a single core on dual core or even a quad core. Although you definitely will not be unleashing the full potential of the server.
As far as performance. Litespeed running one core is still faster and more effecient then apache running 8 cores even. If you serve a lot of static files and static sites then it will be very important to purchase the multicore license on a multicore machine.
I have tested all variations of this and I still find its best to run the multicore license if you have a multi-core machine. Now I may be wrong but in my experience there is a noticeable difference if you have a busy or attack prone server.
What i do basically is, setup a new server which is usually dual ore quad core. Then I start out with the single core license and then when everything starts filling up and getting busy I then go for dual core.