Do you guys think the doctrine is actually useful? If so what exactly should it be used on? Right now the questions tagged with it pretty much look like alphabet soup.
I do not think this tag is useful. If you want to tag a question doctrine figure out what doctrine you are trying to talk about and then tag it with the appropriate elements or doctrinal tradition it comes from.
A generic doctrine tag is useless.
Using tags as a folksonomy as in Shog9's answer, I can see what I think are valid use cases for the tag. We might easily have an expert on different doctrines of justification, and to them the tag combination justification doctrine would help searching for interesting questions.
In the general case, tagging by the name of the doctrine won't work, as in all cases the different doctrines don't really have names but are rather characterized by the name of the church they are used in. We can't really have lutheran-doctrine-of-justification.
Now, I will admit that my own use of the doctrine tag has probably been bad, as has my use of tags in general. I will try to correct my ways.