You may want to disable Spotlight as it can run amok if when indexing. For example, I edit a lot of video and regularly have different drives attached to my Mac. A RAID array, a few drives I have shot from, and maybe a USB thumb drive for output.
I will also have Final Cut Pro open and potentially Parallels open as well. The computer will often become slow to the point where it is unusable. I fired up Activity Monitor and saw 15 or so instances of mdworker. The Mac was trying to index multiple drives while I was already doing heavy CPU intensive activities. I killed Spotlight (which I don't use anyway) and the system is fast and happy again.
Anyway, that is one reason why you may want to turn it off.