Some folks may want an AR for a range gun to shoot a couple of hundred rounds per year from a bench. Nothing fancy, just some range fun. No rough handling or high stress situations involved. For this circumstance a polymer lower may be a cost effective solution to AR ownership and will probably work just fine. It can always be easily replaced if it breaks.
For myself, I prefer a more durable working firearm capable of doing the whole thing as the platform (M-16/M4) was intended. The extra $150 to get that level of performance is well worth it to me.