ele shammy, demo, and desc don't suffer in the same way as moving as a spriest. Those dots you mention don't tick hard unless you have high voidform stacks and those high voidform stacks are only maintained by the rooted casts aka flay and blast, so overall you can be moving around more on a shammy with wolf form, wind totem, and gust and lose not that much dps in comparison to shadow. Warlocks also have more choices to move around and faster then spriest does with the pitiful body and soul, warlock circle, portals etc etc...overall if you mess up a slight moment on spriest it hurts more then desc or demo.
http://www.wowhead.com/artifact-calc...shadow/AjUTIvA look at the gold traits.
On the topic of this thread though, many people have said that when casters could cast on the move it lead to raids litterally not bringing melee outside of the min needed for a mech because if you can move 30-40 yrds around the boss at a whim then there isn't much downside to that, hence why since the dawn of this game till the anomaly that is legion most raids have stacked ranged.