It's not only the amount of desert tiles to make it worthwile. It's also the importance of that area. That is in fact a much bigger factor to make it worthwile. The desert in RFCE hasn't got any importance. Therefor, it's not worthwile to include it, even though desert covers up a decent part of the map.
In fact, it would be great if we could remove the desert altogether, leaving only the African coast. (I mean, you can't do anything in the desert. It won't change anything if we remove the Sahara) But that is technically impossible because we need a rectangular map.
Why addinig something to a feature, you prefer not to have at all, but can't remove it because of technical reasons?