Beyond the fact that it works non-destructively on the RAW file, an advantage in its own right, the Aperture version uses some trickery to blend in actual grain for an extremely convincing effect.
The Lightroom version, however, has access to the camera's profile settings for an even closer emulation to film (Aperture doesn't allow this). But because LR has a "grain engine," it uses that. It's just noise, of course, whereas in Aperture it's real grain.
I find the tool very useful -- maybe not in most circumstances, but in many. Enough to justify the purchase, certainly. And since my plugins like Nik and Alienskin and the like all take my files up to and over 100MB, anything to get a handle on that problem is welcome.