To prevent yourself from loosing cast time while killing mobs, is also known as "Power Word: Shield".
And how often do you
have to kill an elite?, just a few quests. With some brain you should be able to do that anyway without Improved shadowform, or with the help from someone else.
To survive on a pvp server, Improved shadowform is a nice help. But at the time you get dispersion, you can live without it:
PW:Shield, MB, dot, dot, dot, dispersion,(dot's ticking more than half its hp), finnish it of.
Just my point and how I did it on both my priests, both on pvp servers.