Two years ago I wrote a post titled Are you an ambitious person who isn't big on goals? where I proposed that 80% of ambitious people are goal-oriented and 20% of ambitious people are not. See the very intelligent comments to the post.
I'm in the 20%. I have few specific long term goals. I have no idea where I'll be in 10 years.
I'm not opposed to making long term goals and occasionally I do so, but I think there's an underexplored danger to them. When you hyper-focus on a long term goal or plan you become blind to new and random opportunities which may come up as you get older and acquire new experiences and as the world changes.
In a recent article in the Boston Globe titled Why Setting Goals Can Backfire Drake Bennett discusses this downside. He also notes the psychology research around people losing intrinsic motivation or enthusiasm for tasks when the tasks are tied to goals carrying a reward. (E.g. Earn a $100 bonus if you achieve the goal of boosting sales by 5%.)
Overall I support goal setting in many situations. But the "importance of goal setting" is like "never give up – persistence is everything": a widely-accepted business maxim that is more complicated than it appears and entails risks and traps if you're not careful.
(hat tip Tyler Cowen for link)