I figure we get two things out of it:

• Regaining the moral high ground; we are no longer complicit. This is, to a big degree, playing for a domestic audience, and I think it's what a lot of protesters are after. Yes, Congress may ultimately cut the cheques, but I'm pretty sure the administration can find ways to tie up delivery of support in red tape.
• Israeli impunity has always been backstopped by the assumption the US would never turn on them. If other countries turn up their nose, it doesn't have the same meaning. Losing American support would be a huge shock to their political system.

Alternatively, we could go to the point and publically declare what everyone knows-- Netanyahu is fanning the war because once it's over, his administration is defunct, and his legal problems resume. We could singularly demonize HIM as a warmonger-- personal sanctions, supporting his prosecution for war crimes, or classic Cold War style encouragement of regime change.

Yes, whatever we do, we piss off Israel, but if we don't take off the kid gloves now, then when? If they finally admit to nuclear weapons by dropping one?