I think it would add a lot to the Apple experience if they provided unlimited iCloud storage for your photos. It would be a much more seamless experience for a product like this.
Google allows me to back up unlimited "standard size" photos for free.
I also don't really see the problem. Very few will use excessive storage, which is the insight those free services with sizable storage is based on, like Flickr with 1 TB free which is essentially "unlimited" when it comes to photo storage. Key is to restrict it to a certain use scenario / file format, which will make it even harder to abuse.
Google allows me to back up unlimited "standard size" photos for free.