Polymer guns just cost less to produce; it's really that simple. It's not because they expect them to be more rugged or last longer. They can be very accurate, at least when new. Many guys buying pistols these days end up shooting silhouette targets at indoor 7yd and 15yd range and are happy with that.
I doubt you'll see many polymer pistols out in the hunting fields, or on the 25yr/50yd line at bullseye matches though. They do, however, make acceptable holes in things at close range which is what most pistols are used for in any case.