I'm playing as the Moors right now and I sympathise with your point, although I tend to use a mix of arab cav and christian guard. The infantry castle units are totally uninspiring because theyr'e all damn spearmen in various guises. Nubians and Berbers even look the same, have the same stats and follow each other in the tech tree.

So, I've decided to mod the currently unused Sudanese Tribesmen unit into the Egyptian and Moor barracks on the castle side, removing Nubian spearmen from the Moors in the process. This should hopefully spice things up a little and give the Moors and Egypt a unit that will help them get Swordsmiths guilds.... :)