When I first started knitting, I bought yarn to "stock up" on, so I'd have some when I wanted to knit. For a while, that was good, as I was mostly knitting little tiny things like a camera case or a little tote...little projects to learn cables or stitch patterns, etc.
As a result, I have 1 or 2 skeins of a bunch of Bernat Softee Chunky and Vanna's Choice, a bunch of novelty yarn, all in a variety of colors (none of which seem to complement the others!).
So, now that I've been knitting for a couple of years, I find that there are certain yarns I like better than what I have, and I'm feeling guilty for having such a big stash of stuff I don't have any particular patterns for...and which I don't have enough of any one color or style to do much of anything (yes, I bought One Skein Wonders! :-D)
All that to say...it's fine and good to buy a few skeins of stuff and wait till you have a pattern for it--but don't spend a fortune that way, or you'll find yourself with a ton of yarn that's not what you want, but you feel to guilty to buy more with all that just sitting there.
So, for the moment, I'm working from stash or buying for specific projects!