[edited by: Sunnyvale at 9:21 pm (utc) on July 31, 2006]
But in some areas, you cannot afford to leave any opportunity, however small, to competitors. If every remotely similar doamin is in sue, then your investent is probably worth it.
If, as with many of my sites, the .biz, .info have never been used, then chances are I'd have wasted money even buying .net
And for the sake of completeness, if you were using, say, .info, then I'd ALWAYS buy .com, even if only to 301, as .com is the 'default' for many searches and browsers, and so would ALWAYS be a risk if not secured.
But if you have and use .com, then it all depends on the competitiveness of your niche.
I never make the same mistake twice.