Write a Synopsis When?

I’ve heard and read that writing a synopsis for a story before writing the first draft can sometimes be helpful. Last week, as I carved out a synopsis for an almost finished work in progress (sculpting a slab of granite with a butter knife would have been easier), the wisdom of writing the synopsis ahead of the story made more sense to me. If I had some sort of synopsis sketched out based on my original intent for this novel, I would have had a launch pad of sorts. Believe me, I would have killed for that last week. photo by Clarita at www.morguefile.com
I am also wondering if sketching out a synopsis ahead of time might be a way of outlining or feeling out if a story idea has enough potential to morph into a full fledged novel. A pre-story synopsis might help a writer figure out her intentions for a story, maybe help her figure out the merit and meat of an idea, along with its perimeters. Yet, without the commitment. The story could still change during the writing. After all, what a writer needs to craft a synopsis is what he or she needs to put together a first draft: The protagonist and his or her goal, the antagonist and his or her goal, the protagonist’s internal or emotional conflict, his or her flaw, and the setting.
Okay, I know that there are many writers that work out the story while writing. I am impressed and amazed by this, by the way. And yes, each writer must figure out the way that works best for his or her story and muse, but maybe sketching something out ahead of time could be useful and motivating, even if just as a warm up exercise. 
What do you think? Are you scoffing at this idea? Or is it, at least, a little bit intriguing? Or, maybe more than a little bit intriguing? Do tell!