Depends on the video content I suppose. If it's something expendable like gaming content, then yeah I could see deleting it after it's posted. But for me, I've been vlogging, and while I edit hours worth of footage down to five minutes each week - there's lots in those hours I wouldn't want to throw away. I may never use it, but I like having it in case I do.
If there's a question on whether to keep it or not, then keep it. Just invest in some cheap 2-4 terabyte external hard drives to store it!