Oh, very true: Those times I've had to crit a workshop piece that was made up of excerpts from the middle, my *first* complaint was the inability to really see the arc, and therefore the inability to judge the merits of the story as a whole. (I did it myself once, and discovered from the other side how useless it was, too. I blame the fact that I was 19.) I can't imagine handing in an ending usefully to a workshop.

I just find that it's so easy to get crits for the opening, and to keep rewriting them until they're dead on the page, scoured into nothing instead of polished.