There has been a recent change to the Dark Exchange ability for sorcerers (both stam and magicka) that is discussed in other threads and I recommend reading these discussions for players concerned about how the changes affect gameplay.
I wanted to share a discovery I made, which I feel, other magicka sorcerers might find useful in both PvP and PvE environments. I chose the dark deal morph (converts magicka into health and stamina) for the base skill "dark exchange", as opposed to dark conversion- which is a morph that converts stamina into health and magicka. I tested both morphs on my magicka sorcerer, and feel the former has much more utility for my playing style. Since my stamina resource pool is already low, using rapids drains most of my stamina in one cast. My stamina recovery is also low. The only time I slotted rapids was for riding my mount (using an addon that does it automatically and removes it once you dismount, if not in combat).
Now, I slot both dark deal and rapids on my back bar as an essential and now economical skill set combination used in a variety of applications. I imagine other players might not want to give up two slots on their back bar in exchange for extended enhanced mobility, but it's nice to have for dismounted situations like farming, pvp play, and skyshard hunting. Of course sorcerers also have abilities like bolt escape and boundless storm to boost their speed for brief periods of time, but I like being able to use dark exchange in conjunction with rapids for the extended Major Expedition buff that rapids has. I have not tested it to see if using dark exchange with vigor is very helpful, but that is another potential application.
The downside is you lose the ability to exchange stamina for health and magicka. If your magicka sustain is very good, which it already should be for a magicka-based build, dark conversion is rarely ever needed. Also, with such a low stamina pool to begin with, dark conversion is only good for a few casts.