Back when Spotlight was first introduced, it was a somewhat buggy incarnation that many people found to be too much of a hassle and performance drain. Others chafed at Apple's Nazi-like tendencies in its insistence on indexing all of your data--files, emails, everything. So people disabled it.
In the years since, though, Apple has worked out the kinks and it's become a truly background operation. Still, on my 500 MHz G4, I like to eek out as much performance as I can, so when it comes to choosing between disabling all search functions and saving maybe %1 of my RAM and a sliver of processor speed, the answer is a no-brainer.
But how can you live without search? How will you know what's on your computer, you ask?
See, we used to have these things called folders. For those of you too young to remember, folders are an organizational tool. I know this sounds archaic, so please, bear with me. We'd create something called a folder and give it a distinctive name so that we'd know what went in it. Then we'd put files into the folder corresponding with the attributes of the file. For example, put pictures into the "Pictures" folder and manuscripts into the "The Universe is Laughing At You" folder. We even had subfolders inside of folders, and so on and so forth. For further reading, go here.
So now that I've bestowed the light upon you, disabling Spotlight in OS X Tiger is easy. Just edit your hostconfig file by typing in Terminal:
sudo nano /etc/hostconfig
If the line doesn't exist, you can just add it. Then save file and exit.
Then you want to type in Terminal as separate commands:
mdutil -i off /
mdutil -E /
You have now disabled indexing and deleted the current Spotlight index. And with the edit to the hostconfig file, you will ensure that it all doesn't pop up again after you restart. You kill it and it stays dead.
Now comb your hair back, upturn your collar and put on your shades. You're ready to burn rubber.
Leopard users go here and scroll down for the great debate.