They are necessary because as soon as someone decides that S3 is a filesystem, they will look at the other cloud "filesystems," notice that S3 is cheaper than most of them, and then for some reason they will decide to run giant Hadoop fs stuff on it or mount a relational database on it or all other manner of stupidity. I guarantee you S3's customer-facing engineers are fielding multiple calls per week from customers who are angry that S3 isn't as fast as some real filesystem solution that the customer migrated from because S3 was cheaper.

When people decide that X is a filesystem, they try to use it like it's a local, POSIX filesystem, and that's terrible because it won't be immediately obvious why it's a stupid plan.