We love our friends over at Business Insider (which until a few months ago was known as Silicon Alley Insider). But occasionally we wonder if the pressure of filing 85 items a day clouds the judgment of their finger-sore contributors.

On Tuesday evening, BI’s Dan Frommer published a post saying that the planned public spinoff of voice-over-Internet-protocol phone provider Skype from eBay (EBAY), announced that afternoon, might be worth as much as $3.1 billion. He based that on a note from an investment banker predicting that Skype might have pre-tax earnings of $156 million in 2010 (an aggressive but not absurd estimate). The banker then multiplied that figure by the not terribly precise range of 10 to 20, and came up with the not terribly precise range of $1.6 billion to $3.1 billion.

Why is that number so laughable? We already know that Skype today is worth nowhere near that much money; eBay would not be taking the IPO route if it could fetch even half of $3 billion for the company that eBay purchased in one of Meg Whitman’s more questionable moments.

But ok, IPOs have been known to get overheated. And even if the IPO market today is about as dead as a shot pirate, it will bounce back some day.

Still, given today’s slump, how can anyone justify the 10-20x multiplier? Certainly not by rational projections of Skype’s future growth. The service boasts some 400 million users, but as Om Malik argued pretty definitively in January, Skype’s growth is clearly flattening. Perhaps most importantly, Skype’s most important selling point is that it’s free between Skype users. If Skype as a standalone company tries to charge users, millions of them will simply switch to a free competitor. The fact is, standalone VOIP companies–even when they are able to charge for their services–don’t have a great track record. Just ask Vonage (VG).

Don’t get me wrong, the eBay plan is smart: Get Skype off the books. But eBay’s wishful thinking about Skype’s value was wrong in 2005; any investment banker arguing it’s worth $3.1 billion today or next year is just as wrong, and ought to be quizzed, not copied.

UPDATE: The New York Times has found an even more credulous analyst to say Skype’s IPO could fetch as much as $4 billion! Hey, why not $10 billion? Doesn’t anyone ever ask these people to define and defend the valuation yardsticks they use? Comparisons to past IPOs have little use since, as the Times at least acknowledges, “public markets [have] not been very receptive to initial offerings” of late.