Medical device specialist DexCom (DXCM -0.14%) is going through a rough patch. The company's shares are down sharply year to date due to unimpressive financial results -- and some of the headwinds it has encountered are, no doubt, concerning. The company's slower-than-expected growth in international markets in the second quarter comes to mind.However, there remain excellent reasons to be bullish on DexCom's long-term prospects. Let's consider three of them.1. The power of CGM systemsDexCom is a leader in t ...