Is there really any reason to use Leopard?

My 2Ghz Macbook tends to slow down a lot (it will be alright most of the time, but sometimes will slow to a crawl with no warning), and I have a hunch that it does so at least partially because it’s bogged down with running Leopard. So, I was thinking of reinstalling Tiger and stripping off a lot of the extra crap (namely, applications I never use) to speed things up a little bit.

So, my questions are:

a.) Are there things I need Leopard for besides Time Machine and Spaces?

b.) Would installing a stripped-down version of Tiger have a noticeable effect on how fast the computer runs?