Not to enter the Google v Dropbox argument, but with data caps, it's easy to slowly build up a lot of backed up data that you cannot really retrieve all at once.
Not to enter that argument either, but whatever cloud storage service you use, you should be storing your data there as well as on at least one local hard disk, not instead of.
And yes it can take a long time, but you might swap cloud providers every 2-5 years, you're not doing it every day. Even if it still takes a month to copy all your files, that's generally an unattended process, so it's not such a big deal.
I'm not saying it's trivial, but it's not a huge deal. All things considered, it's pretty easy to switch.