I guess it seems fairly sensible to to me...especially when people get introduced to it as print unspaced [...]. If that's how you learn it, you then think of being able to lift out the unspacing behavior you used when you needed it into a function. And you can say print spaced [...] too, though that's usually redundant.
Though if it were just JOIN it might appear clearer in this particular case, I think reserving JOIN for something else makes sense. Trying to use a generic series-joining operator for string production is problematic, as @Brett explains in detail.
But it's a good point that presenting FizzBuzz probably needs a little lead-in to explain UNSPACED to the uninitiated, to make absorption smoother.