If you want to be a sophist you can archly suggest that specific searches (as described below) could just as easily have come from 3rd party referrals, an advertisement, etc. Fair enough. But if you're going to play the Devil's Advocate role, then I am going to ask you if you're taking the time to ask THEM?
Your website analytics can already tell you HOW the visitor got to your site. It is incumbent on you, the marketer, to parlay that knowledge into MORE knowledge.
"How did you hear about us?" is the most under-used phrase in marketing. But even when it is used, rarely do marketers INCENT the visitor to take the time to (accurately!) fill out a brief online form.
You gotta give something to get something.
How many times have you seen a Dell ad on television that suggested you visit www.dell.com/tv? They want you to visit the "/tv" site so they can measure the effectiveness of their tv ads.
But, who the heck does this? As a typical consumer, I'd just go to dell.com to configure my new PC. I may indeed "arrive" at the site thanks to the ad, but Dell's ad firm gets no credit.
Here's a variant of the magic phrase to add to these ads: "for special promotion pricing, tv viewers should log on to www.dell.com/tv."
OK, I did that. Looks like I get a special deal on the A920 All-In-One Printer: $9 off the MSRP of $89. But wait. Let's blow out the ol' cookie file and return to dell.com. Search for A920 printer. Lookie, lookie: any schmoe who visits dell.com gets the exact same nine bucks off as the schlep who followed directions and visited dell.com/tv.
You know what else bugs me? A too-clever-by-half websurfer might tap in "dell.com/tv" hoping to bypass the main Dell site for info on those slick new Dell TVs that have hit the market. The websurfer would be misguided if they truly thought that this surfing strategy would work, of course...but on the other hand, would it be too much trouble for Dell to add a hyperlink on the dell.com/tv page that said, "if you're looking for information on Dell televisions, click here"???