Michael Baron over at Metsblog had a good take on why the Mets would want to trade Jon Niese. While Michael makes some great points, to me, the fact that they're looking to trade him right now is actually a testament to how valuable he actually is. For the same reason the Oakland A's are shopping Gio Gonzalez, Sandy Alderson thought it was a good opportunity to listen to offers for Niese. This action is completely dictated by the current state of the market for starting pitching. There is a ton of demand (as there always will be), but there's little supply. A young, cost-controlled, improving, left-handed starting pitcher holds heightened value right now because there aren't many available, quality starting pitchers in trade or free agency.
One thing that should always be kept in mind is the fact that the current team of executives running the Mets baseball operations is very savy, and they would never make a trade they didn't whole-heartedly believe was in the best interest of the franchise long term. With that in mind, I can say with certainty that Niese will not be dealt unless it's for a package of assets that makes the team better off long term. The Mets obviously feel that they could get incredible value for Niese if they deal him now. And I'd have no problem with trading Niese in a deal like that.