Stronger user lock in once they convert users to GDrive. If your plan is to lock users into Google services, then having all their files in GDrive, integrated with Gmail, GDocs, G+ etc. is going to work towards that. Plus it allows them to corner a market as I suspect Dropbox easily has a majority (I'd guess 80+%) share in consumer cloud storage.
Do I think that is worth what Dropbox would cost? Probably not, but Google has heaps of money and hey, what would I know?
Do I think that is worth what Dropbox would cost? Probably not, but Google has heaps of money and hey, what would I know?