Read through this wiki:
http://en.wikipedia.org/wiki/Unsprung_mass. It will give you a great overview in the benefits of lighter/smaller wheels, besides that of costs (which is true as well).
Here's another link that can explain things a little better:
http://victorylibrary.com/brit/M-sprung-c.htm
Basically, it plays a large role on your suspension while in a track scenario. This should be the main reason that people are doing this, in addition to just the cost benefits.
To the comment on unsprung weight: it's not unsprung weight that helps with acceleration. I believe what your talking about is rotational inertia.