Because this is just a test, the new tier may not be available in all markets and it is uncertain whether or not Netflix is going to offer it everywhere. We probably wouldn't want to suggest that those who view Netflix on a tablet watch streaming videos in SD, but for those using a smartphone, you might not even notice the difference.
Another thing to consider is that competitors like Hulu Plus also charge $7.99 a month and this is a way for Netflix to undercut them.
Netflix is testing a cheaper $6.99 monthly plan
source: Netflix, TechCrunch via AndroidAuthority