one can argue that you loose time to dps from running. Let say you loose 8 secs per shield phase, 4 shield phases, so a total of 32 seconds total in the fight... depending on how much dps people are pushing out walking of flying can mean the difference between death to an enrage or a kill. With that said though people shouldnt be killing themselves to dps; meaning if for what ever reason you're sitting at 50% life and DD on you, you should be poping a CD and running.
Another good point, but for example on stormcaller when the shield is at the other side of the door when getting knocked off you can get stuck in a tree and die. Walking worked fine for me as a sentinel who was taking double destruction and i still managed to pull out 1900 dps in the full fight and 2k dps on the bosses themselves. So if the players are somewhat competent walking is no problem.