Hi MD9 , the reason for this requirement is that we want to make textures here on 3DOcean as useful to as many different industries as possible. There is a lot of free texture material available out there that sites like ours need to compete with and so putting the extra effort into making a texture for sale that will save the user a step or 2 makes a lot of sense.
If texture sizes are not to the power of 2, you will be able to see borders in game engines that generate mipmaps because borders will blur. It will keep mipmaps crisper all together. It is also important for optimizing real time application performance. Some even require you to conform to this rule.
Also if you have a UV mapped model and just want to throw a texture on, you will need to adjust UV coordinates by hand to counter the texture skewing and stretching effect if the texture does not conform to the power of 2 rule.
This does not mean that you always have to have a square texture. You can have textures that are 1024×4096pixels for example. Adjusting UV coordinates for power of 2 formats will still be easier then doing it by hand because you can simply scale them by increments of 50% in x or y direction. Its just a much more accurate method.
HDR images are only used for rendering where this requirement is not as crucial. If this was for a game sky dome, we would still reinforce the rule.
I hope this makes sense and you can appreciate why we are being so strict about it. There is a lot of information on the internet about why the power of 2 rule is around and lots of reasons why sticking to it is a good practice if you have the time to read up on it.
Sorry for the inconvenience this has caused you. You are of course welcome to include your current textures and source material in the download file! I appreciate the work that has gone into your textures and I hope you won’t be discouraged! 
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