A very wide stance makes it very hard to move and it isn't good for infighting, either, if by that you mean throwing punches from close range. If you are looking to throw sharp punches that travel 6" or 8"...well, you can't do it with your feet far apart because your weight won't get turned through. Watch the Chavez/Rosario fight: Rosario kept his feet wide and thus needed more room to punch, and Chavez, with a proper stance, was not only able to move in and out, but he punched the hell out of Rosario with short hooks and uppercuts.
Dempsey stepped in real hard with his straight left- it wouldn't be fitting to call it a jab- and, when you are looking to get inside, stepping in with the jab (by pushing off the back foot) is a good way to do it. Remember to bring your back foot in with you. The jab only needs a short turn of the shoulders and a very slight turn of the hips, unless you are very squared up, in which case the jab will be slow and the right hand will be another jab.
Many years ago, at this gym where I would take my fighter, there were two guys that everybody would stand and watch hit the heavybag. One guy was short and went 230, the other guy was tall, maybe 200, and they'd whack at that bag. Make all kinds of noise, spin in place, but not move much. My kid was 14, 115 pounds, and he'd pratically knock that thing off the swivel with his jab. He punched real good, good balance, and turned his hips.