Worth it or not worth it, it's awfully late to pull the plug now or all that money already spent on the aircraft would be thrown away. Besides, the USAF actually does need a new fighter. Need I remind you that the F-16, which the F-35 is supposed to replace, is actually a 40 year old aircraft?
First off a disclaimer - I'm not American. I'm actually Swedish so I have a natural bias for the Saab Gripen which is a competitor to the F-35. I'm not a big fan of the F-35 either way - for most countries it's excessively expensive to both buy and maintain, especially since few actually need 5th gen aircraft.
Even so I think it would be incredibly foolish of the US to pull the plug on the F-35 because it leaves you with three options: either start another expensive tender that will likely take 10-15 years, restart the production line for the F-22 which is even more expensive and poorly suited for anything but air superiority or drop the F-35 without a replacement tender for the F-16 which is absolutely insane - doing that is essentially turning the USAF into a second rate air force and forfeiting the air power game entirely to Russia, China and anyone else who buys Russian or Chinese 5th gen aircraft after 2020-2025 at the latest.
So unless you'd like to dissolve NATO and become isolationist again, you guys are stuck with the F-35 for now. Don't feel too bad - it's a perfectly capable aircraft. You, along with the rest of the partners, are just going to have to pay more than expected for them.
EDIT: Oh, and as for "limited usability" - you're not thinking every conflict from hereon out is going to be like Afghanistan, right? Unless you haven't noticed, Russia and China have begun to flex their muscles quite a bit lately. The post-cold war lull is over. Hell, the one with Russia is practically back on now that even cooperation around GPS and the ISS have been put on the line. If you want the US Military to remain a credible deterrent that means they need 5th gen fighters and the F-35 is your only choice right now.