I was hoping you would get a reply from someone who has stayed in one of these cabins, but since you did not. Here goes. Keep in mind that although I have recently returned, I did not stay in a cabin like you are asking about. That said, I did see them. They are mostly below the water line on the boat. Those half-windows sit just above the water level and when we were docked, those windows were indeed below the dock looking into a concrete wall if that was the side you were on, otherwise looking out at the water level. They are also smaller than the upper level cabins and I did hear a couple of singles that were in them say that they did not think the cabins would be big enough for more than one person. I guess it depends on what is important to you in a cabin, but there are ample reasons for the much lower prices on these cabins.
The fares for cabins on nearly every ocean cruiser and riverboat are determined by their location and their amenities. While cruiseships break cabins into as many as 40 different categories (don't forget--megaships may have 1600 cabins), the majority of Viking's riverboats have four categories (though the Russian riverboats have six categories, while the Schumann and the Normandie have only three).
Here are some details on the 4-category riverboats: The top-deck cabins, category A, are the most expensive. Not only do they have the best views, they have large picture windows that open. Category B cabins are usually on the middle deck of Viking's ships. The view is deemed to be not quite is good, but the cabins are generally the same size as the A's and also have large picture windows that open. Category C cabins are usually a full deck lower than the A's, sometimes two decks lower. Usually the C's are the same size as A's and B's and have large picture windows, too; but because the windows are closer to the water level, they do not open. Category D cabins are on the lowest deck and are the smallest cabins. A river view is offered through one or two portholes. D's are usually marketed to cruisers travelling solo. In fact, while A's, B's, and C's are available to solos at 200% of the double occupancy rate--as is the custom on cruise ships--the D's are offered at 150% of the double-occupancy rate. (On Viking's riverboats where there are only three categories to choose from, the C's are usually offered at 150%, rather than 200%, of the double-occupancy rate.)
As dfb tacitly pointed out, the accommodations you choose ultimately depend on your needs and wants, usually as governed by your budget.