Andy_Langton - 11:49 am on Mar 29, 2012 (gmt 0)
Further to phranque's comments, it's perfectly possible to rank a robots-excluded URL, and if needed Google will take the word of third parties as to what content it contains - primarily anchor text.
Links alone can be more than sufficient for many queries - particularly longer tail ones. But those can also include smaller brand names and other searches which are important in their own right - even if they are not particularly popular or competitive.
There's also a difference between a URL that has always been robots excluded, and one that Google has already captured, but is excluded at a later date. Google keeps history on URLs which can still impact on them after they are excluded via robots.txt directives.