Didn't you start with the original iPhone? The metal-backed original iPhone is as slippery as the 6/6+ so why the sudden change?
Not necessarily. Some cases can do more harm than good, especially if stuff gets in and you don't remove it. See http://forums.macrumors.com/showthread.php?t=1840854more resale-value (because less scratches etc) if you want to sell your phone and buy a new one.