Backup is about having several copies. If you delete files after you uploaded them to the cloud, you don't have a backup - with all the problems that come with it (say, if you delete a file by accident there will probably be no way to restore it).

If your files are important to you - you need a back up. If not - just get rid of them in the name of simplicity :)

PS. Also, uploading many gigabytes over a typical ADSL connection is just painfully slow... even local NAS is too slow over WiFi for my taste, and if I have to have a wire - I can just as well connect an external drive.