Where is the Photo Stream greyed out? In iPhoto on your Mac? Online? ???
This is from Apple's document iCloud: Purchasing iCloud storage and billing
iCloud customers are provided with 5 GB of free cloud storage. Purchased music, movies, TV shows, apps, and books, as well as photos in your Photo Stream don't count against your 5 GB of free storage.
It's used for:
You can change your iCloud email account preferences (under Mailbox behaviors and Advanced) to not store deleted messages online and other changes to reduce the space usage.