For example: "...at the same cost as a regular printed book." Yeah? How's that work? Remember that instead of trucking industrial-sized barrels of ink to printing plants, you're trucking smaller amounts to thousands of bookstores. Same for paper. Same for cover stock. How's that going to be the same cost?
The presses in a printing plant can turn 24 hours a day, and earn money 24 hours a day. The POD machine is used ... when? And the rest of the time it's just a hunk of metal that cost money, but isn't bringing any in.
POD has no economy of scale.
How much time does it take to print a book? It's got to be non-zero. How many machines does this bookstore have? What's the Christmas Rush look like? What do they do when their printer breaks down?
Hmmm, much as I hate to disagree with you, Uncle Jim, I think that POD does have a place.
You seem to assume that a POD printer is standing somewhere in a bookstore and just used to print a book a day or something. In reality, these are centralized just as any other printing press. The main difference is that offset printing produces huge numbers of the same book quickly and cheaply, while a POD press can switch quickly and efficiently between printing different books.
Where POD works is where you eliminate the bookstore or warehouse in the middle. The customer orders a book, the book is printed and shipped directly to the customer. This saves on transportation (from the printer to the warehouse, and in some cases from the warehouse to the bookshop) and in theory it also means that there will be no unsold copies that will have to be pulped.
This will become more meaningful in the future. Americans are used to low fuel prices. Mobility is a way of life. But even in the US, gas prices have been rising. Will those huge numbers of books still be able to drive around in trucks so cheaply in 10 years? In 20? It seems that transportation cost will become a major factor, at least it will be a bigger cost factor than today.
It's also not inconceivable that paper will become more expensive. Bear with me here. Imagine that oil does become scarcer. What is one of the proposed alternatives? Bio-diesel. But this requires that a lot of biomass be grown. This will compete with fields used to grow food, wood, and everything else. Granted, paper can be recycled fairly well, but it may still be a factor.
Last but not least, Offset printing has been done for a long time. POD is a new technology. If both decrease production cost by, say, 10% this has a larger effect on POD due to the higher base price. POD will (probably) never reach the low prices of mass offset printing; but it will get closer.
Another criticism I read is "quality control". While it's true that any garbage can and does get published as POD, this doesn't mean that POD is inherently unviable. POD is a printing technique, nothing less, nothing more. I don't see why there couldn't be a publishing house that does exactly what current publishers do, then produce as POD. If they do a good job, consumers would trust this POD-Publishing house just like they would trust a "classic" publisher.
It may even be good for authors in the long run. The largest share of a book's profit is kept by a bookstore. POD is a means to completely cut the bookstores out of the distribution chain. This might mean that even if you move fewer copies of your book you end up with more money.
Time, as always, will tell.