I had Jehovah's Witnesses with HGB as low as 4.8, which is crazy low, but they were absolutely not surgical candidates (they were medical patients only). Iron infusions do not give volume, and if you're bleeding out, you need packed cells, plasma, and cryoprecipitate. Faced with a low hemoglobin, dumping loads of *just* fluids in you will not fix your problem, IF you are faced with life and death hemmorrhaging. It will just dilute what you have, and have its own set of issues (certain fluids in bulk can increase acidemia, etc.). I think I DID read JW can accept cell saver blood, as Jan said, but the physician would have to be in the know and prepared for that with you.
And yes. Truth is if you WERE a trauma patient who came in hemmorrhaging, you're going to get blood. Even if you have an advance directive, they might not know, you might not have a card on you when it happens, etc.... though it's still good to get that in order, just in case.
Spooky's answer is pretty much what I'd say, otherwise. I agree with her.