If you think of a normal Data Warehouse in the old SMP fashion as a big humpback whale and the new Parallel Data Warehouse version as a pack of killer whales I think you clearly get the picture of what Microsoft is attempting to achieve. The old humpback is slow and fat it can only grow to a certain size before it really runs out of steam it trundles slowly through the water with feeding on the smallest of sea life (krill) these are in fact your data loads and returns you result sets at a slow steady rate. Whilst the killer whale pod can grow and grow its very nimble can take large chunks of new data it can respond to request in a very timely manner.
The issue for companies when evaluating PDW versus the old Single Server Data Warehouse is the hardware cost. Now realistically I don't think this is a real issue if we look back on history 10 years ago only the biggest companies where involved in Data Warehousing the wanted and need to get answers from the years of information that they had collected and stored. Today small business is using data warehouses to gain the same information from their data. The amount of information that companies collect has increased and will continue to increase and therefore PDW has a real place in the market place as companies want answers faster and faster MD's no longer want to wait a week or even a day to know what is happening.
The beauty of PDW from an administrators point of view is that Microsoft has built in redundancy at every level, the management console monitors all the servers in PDW to alert the admin's to issues and problems that are affecting the system.