Almost all of my TPU tubes are the smooth plastic and my Topeak JoeBlow floor pump clamps on just fine. Ditto for my Lyzene frame pumps (though in honesty, they screw on to the short threaded section of the actual valve). Why not metal stems? I am guessing cost; weight savings as well. My many decades of experience is that the lock nut on a Presta tube is superfluous on a road bike.
John