Releif buys you exactly 5 turns. This can be huge with communion since your mages can deal great damage in those turns. However it is not a panacea. Further in large battles with many Thurges one releif caster is no where near sufficient. Even massive numbers of communicants can accumulate huge amounts of fatigue per turn when there are 12+ thurges casting.
And in a shocker to me it is actually more effective to go for evocation than for Thaumaturgy when it comes to Thurges. I was trapped in the Pythium is an astral nation hence go for Thaumaturgy for battle magic. With a good number of communicants ( and you will need a good number ) some of the early Evocation spells ( and the computer will even cast them! ) do a great job.