I realize both flag types mean something completely different, but do we really need to have a separate flag type for them?
I think this kind of answers your own question.
Having distinct options is a really simple/lightweight way for a human to give feedback to the system as to why the post needs attention.
While there currently isn't a different net result from an end user perspective, it's becoming valuable information for upcoming data mining activities. Of course, there is a certain amount of noise in the system, which should be reduced as much as possible, including encouraging users to think and pick the closest option as opposed to simply always picking one or the other.
By the way, spam and offensive flags appear in the same list in the mod tools, while the "moderator attention" flags appear in a separate area.