MedInf are definetely useful. It allows you to have a seamless transition from Swordsmen war to Medeival war, rather than having to have peace to build up a knight army. In a recent game (where I'm doing very well) I started a war with the Mongols with swordsmen, then finished it with MedInf, then consolidiated the MedInf army and took over Greece. You may say MedInf are too slow for Medeivial combat, but being able to upgrade one strong army to another is very useful. It saves you the trouble of making a sword army, fighting a war, then making an entire knight army from scratch. You could just make horsemen and upgrade them, but that has a steep upgrade cost, and with sword to MedInf, you don't have to make a new army, you just upgrade what you have.
CG