There are indeed 8 & 9 day MYW passes available. If you are going to get the expiring ones, then the price difference is very small between the 7, 8, 9, and 10 day tickets, less than $5 per day.
The real drawback to these 8 & 9 day tickets comes if you want to add the no expiration option to them. Disney has set that option at $100 per ticket for them, which is $45 more than the non expiration option on the 7 day ticket. That means that getting the non expiration option will price the 8 day ticket about $50 more than a 7 day.