It's dangerous for Dropbox because GDrive will be automatically integrated into all Google services. I can access it anywhere from any Google service or device. No need to think about uploading, downloading, copying, pasting--it's just there. I don't use GDocs because it's the best.
I use it because all my friends use Gmail and sharing docs with them is super easy. I see GDrive having the same benefits. Sharing on Dropbox still isn't as easy as it should be. I've been trying to convince my girlfriend to sign up for a Dropbox accounts since we started dating, but no luck. With Gdrive, she will never need to.
That said, success is still not guaranteed. MS has had SkyDrive for years and I have no idea what the market penetration numbers are, but I'm sure they aren't good. Hotmail is no Gmail. I had no problems convincing a bunch of people to switch to Gmail, but only got one person to sign up for Dropbox.