Not that I think Uber is handling this constructively, but there's a valid reason why they would not offer the valuation bandied about in the press: they are not buying the same equity as the $17 billion+ equity.
The venture investors bought Preferred equity, the shares being sold by the employee are almost certainly Common equity. Common equity is worth less than Preferred equity. The times I have seen offers to buy Common shares in connection with a Preferred round, the discount has been as high as 50%.
This makes sense: assume you are an investor putting $1.2 billion into a company that has previously raised $300 million (this is roughly the recent Uber situation.) You are investing at a $17 billion valuation. Now assume that you were very, very wrong on the valuation and the company ends up selling for 90% less: $1.7 billion. You still get your money back. If the company goes on and sells for $34 billion, you double your money. This asymmetrical payout is one reason venture firms are comfortable taking the risk of such a high valuation.
Now assume you bought $10 million of Common shares from a former employee. If the company ended up selling for $1.7 billion, you would get back roughly $100,000 (assuming the preferred owns half the company.) You don't really have an asymmetrical payout: you stand to lose money if the valuation is wrong.
A wider the spread between the Preferred price and the Common price implies less confidence in the Preferred valuation. The actual valuation is the Common valuation, and the extra amount paid for Preferred stock is a kind of option value. So an alternative explanation to Uber short-changing its Common shareholders is that they believe the expected value of the company is closer to $4 billion than $17 billion and the Preferred buyers think the range of possible outcomes has an extremely large standard deviation.