Probably the main reason why I do not mat my 8x10 prints too 16x20 anymore, is because 16x20 is a standard frame size. When selling photography in a standard frame size, people tend to purchase cheep, premade frames. These frames are usually not good quality and there is not much a selection...so people end up putting a beautiful photograph is a cheep walmart frame!
By using 14x17 (or some other random size), it requires the buyer of my prints to have the print custom framed, therefore getting a better quality frame, but also getting a frame that would compliment the photograph and match their home. By doing this, they have more selection and can get a frame that not only looks well with the photograph, but also will look good in their house! Custom frames are usually sealed on the back also, which stops dust and bugs from getting inside.
I no longer use Standard frames sizes for my photography for this reason, but also because there are sizes that I think look better then just standard sizes. I get tired of seeing everyone frame their photography too 16x20, because it starts too look generic to me. Here are my normal sizes-
4x5, 5x7, 8x10- 14x17.