A very good question...here's my opinion.
I had GS for about 3 weeks....found it was better to have meditation so I went back to it and gave up GS (as all healing priests should!!) but I did learn 1 thing : communication is key when using it. To answer your question, YES...let the tank die, but only in the 10 seconds that GS is on him. Before you do this, however, you need to talk to both the tank(s), as well as other healers in your raid group. Make sure they all know that the tank must not be healed in any way during this time. This is used as a last resort method in this way, although it can be used mid fight just to gain time for mana regen or to heal others, in which case you're not letting him die anyway. But as a quick save when things get really chaotic, it works really well. Just make sure everyone involved knows whats going on. I personally have saved wipes using GS in this way, and a few people have asked "what was that" after it was over, showing me that not alot of people who have never been a healer really know anything about GS. Therefore I say again : communication is key!
That being said....drop it for meditation and wait till xpac gives us more talent points to use before picking it up again.