The short answer is that short stem plus shortish bars will make the bike very twitchy. The short stem makes the handling and turning feel very direct and fast which is awesome. One of the reasons for the wider bars is that they slow down the steering just a little but more importantly, it takes the twitchiness away and replaces it with much more control. The next reason for going wider is that the short stem and short bars will make you feel cramped. Stem brings you closer, wide bars stretches you back out so your reach should be roughly the same as it was with long stem/narrow bars. The wide bar/short stem allows you better leverage for all obstacles, wheelies, bunnyhopping, cornering etc. Allows you to have a more central to rearward balance over the bike for riding steep descents instead of over the front (which only makes sense if you ride terrain where you need to be drafting in an aero position). Think of it as doing a push on the ground with arms close together vs wide apart. When you ride a bike over uneven terrain it's much easier to maintain control when a lot of lateral forces are pushing on the bars. Much the same way as your friend pushing you over while you doing a pushup. I ride 800 bars with 50mm stem (have broad shoulders ). I hardly ever have a problem with narrowness. But when I do I just lever or swim the bars through the gap at slower speed. Your trail builders need to catch a wake up and realise trail riders have gravitated to wider set up.
