First, since cloud storage is the only reason Dropbox exists as a company I think they'll try harder to do it right. If Google Drive doesn't live up to Google's expectations they'll shut it down like they've shut down all of their other unsuccessful projects.
Second, possibly related to the first point, it seems the other companies are doing cloud storage to sell their other products. SkyDrive ties in with Office Live, Google Drive with Google+, etc.
Given that Google's charging for Drive and not G+, it kind of seems like the other way around.
There's always a risk a company will either shut down a business (Symbian, Wave, MySpace, Delicious), or go out of business (Chumby), or get bought and have their business shutdown (Sidekick, Drop.io). I don't think you can really know the future enough to make product choices based on the chance that will happen.
The fact that they charge for Drive and not G+ doesn't imply that Drive is more important than G+. Looking at everything they have the last couple of months I would argue it is in fact the other way around. Although longer term and not direct revenue, G+ (and it's ad possibilities) is more valuable to them.
Second, possibly related to the first point, it seems the other companies are doing cloud storage to sell their other products. SkyDrive ties in with Office Live, Google Drive with Google+, etc.