For the casual consumer:
Pictures - 1-2MB/pic (point and shoot cameras) = ~2,500,000 photos
Music - 4-5MB/song (iTunes) = ~600,000 songs
Movies - 650-700MB/movie (DivX/Mp4/480p) = ~4,500 movies
For the prosumer:
Pictures - 4-5MB/pic (dSLR cameras) = ~600,000 photos
Music - 8-10MB/song (Flac, loss-less) = ~300,000 songs
Movies - 4.7-8GB/movie (720p-1080p MKV) = ~820 movies
We can see the high-definition movies eating up a substantial amount of hard drive space eventually, but really? 820 movies? We don't even think we know of that many movie titles in existence (or worth knowing at least), let alone have the time to watch it all!
Perhaps we should just look at it as if we're overstocking on canned soups for an unexpected disaster. With all of the chaos in the world of Internet today, one can never have too much storage right?