I wouldn't say it's preferable in any particular way. To be honest, I use Inkscape because it is open source and I am far more comfortable using it than Adobe Illustrator. Then again, I dabble in vector-based graphic design purely as a hobby, not professionally. It's important to note, though, that many of the effects (gradients, etc.) aren't always cross-compatible between Inkscape and Illustrator, but if your aim is simply to create outlines, Inkscape is wonderful. I have even used it to create fonts of my own (with FontForge).