I kinda like swatches.
If you find a yarn that you really like to work with a lot (for me it's mmmmalabrigo), then it's worth the time an effort to make several swatches. I used to felt my swatches and use the for coasters, but now I take a permenant marker and write ON the swatch the sz needle I used and the gauge. That way when I want to make something later, I can just refer to my swatches to determine what size needle I need to use for what yarn... and it helps, too, because I can see right away if I like the fabric that the gauge produces... for instance, I get 4.25 sts per inch uing malabrigo with size 10.5s (i'm a tight knitter)... but I don't especially like the the looseness of the fabric. When i come across a pattern that calls for that gauge, I'll know that #1 I can get gauge with 10.5s, and #2 that I don't like the fabric. So I'll choose another yarn.
Swatching is one of those things you can do while watching TV since you don't really have to concentrate. It's nice to work on swatching in the middle of a larger project (for me) because they're quick, and I know that I will refer to them later.