It's only an incentive if people know it's an incentive. And I don't think it incentivizes good behavior. Someone could just ask a second question that's marginal or something. Tags are for categorizing information; categories should not need to prove themselves to anyone
Categories indeed do have to prove themselves to everyone. Instead of, say, ten clearly defined tags, you could have ten thousand poorly defined, overlapping, inconsistently used tags. How is that helping the site, or the community on the site?
For example, that's why we eventually added the tag synonym systems, because we ended up with too many duplicate tags that meant the same thing. More duplicate tags meant more confusion -- which tag is the correct one? Which is used the most? Why do we have all these different tags?
That's also why we added the tag wikis so that people could understand the tags, what they mean, when to use them... ultimately, tags need to explain and justify their existence. And if they can't, they have no right to exist.
Before we had tag wikis, the only way for a tag to "explain" itself was in terms of which questions it was applied to. With only one question over a six month period, who can say what that tag means -- or if anyone even cares? It's just a bunch of litter cluttering up the tag system, and every user who types in a tag has to deal with potentially thousands of these orphan tags (we removed 7,400+ on Stack Overflow) matching what they type, and thinking.. "hmm, should I use this tag?"
Giving this a little more thought, it's probably an OK compromise to require the tag wiki to be completed if you want the tag to remain in place, even if there is only one question ever asked with that tag.
This will get rid of the accidental tags, the error tags, and the careless tags -- where people mindlessly spray a bunch of tags over a question because they can. The kinds of users who create poor single-use tags will not be motivated to create the tag wiki for that tag. And for those who feel strongly that a single question tag absolutely must exist, forever, they should be motivated enough to build a reasonable 300 character tag wiki for it if they feel so strongly.
Or, y'know, ask the second question on that topic in a six month period, if it's really so important.