Let's take a step over for a moment to think about what a tag's purpose is. The purpose of the tag is to tell you what the question is about. In a sense, spoiler actually fails this since in most scenarios the tag applies to the nature of the question's content. This is not unlike how genre tags apply to the game, not to the question itself. Thanks to the spoiler formatting, applying the spoiler tag for spoilers in general is like adding a [code-block] tag in Stack Overflow.
In failing to accomplish the primary goal of a tag, some tags continue to exist as a measure of filtering aids. The question then becomes, does this tag succeed where other tags have failed?
I don't think it currently does, mostly because of the relatively unadvertised nature of the tag, which is in part due to our lack of an announced policy on it. I know badp has mentioned several times in chat the current methodology that is seemingly implemented (only when the title is), and the reasoning behind it (so that people can ignore the tag and let us use more descriptive titles that may contain spoilers). However, I don't recall any mention of it on the Meta site itself, except maybe off-hand in a comment. So unless this gets amended, I don't really think the tag has a very good future.
I'm honestly not a fan of a tag whose standard function is to show up on people's "ignored" lists, not even being used to filter out selected questions. But that's mostly for trying to make a consistent tagging system. In the end, if the tag must live, badp's method makes the most sense. We have spoiler markup to handle scenarios that are not in the title.