There is are only a couple of real advantages to having data directly in a list instead of in a database.
1) The format of the value is immediately apparent when looking at the page in DW
2) The values are easily updated and removed directly from DW
3) A small time savings if no database connections need to be made on the page
The advantages of the database are organization and ease of manipulation if you are familiar with working with databases directly and are using a database.
Powerstore uses the technique it uses to prevent data redundancy it also more easily regulates mistakes and cancelations.
Since the orderDetails table already keeps track of the number of items sold and the orders table already keeps track of the date they were sold, the only information necessary to keep track of inventory is the quantity in stock and the particular time that quantity was counted.
This way if an order is deleted inventory is increased and when inserted it is decreased automatically. I think there are often quite a few ways you can overcome the same problem. I see no immediate drawback of using another table... there might even be advantages we haven't thought of. The powerStore is still relatively new, but I have to admit I can't really deduce the exact table structure you are using from your short description so it is hard to comment at all.
What is more interesting to me is: What was the disadvantage you saw in the way powerstore handles inventory that caused you to change techniques?