Mamluk HAs are to be recruited from Sinai with MHB. Then they need to be combined with arbs to be fully effective. "Fast" cavalry can't do nothing with them - they're too strong, and as long as you pin the enemy arbs with your own the Mamluk HAs are free to do their thing. It's the combination of weapon systems that is the real gain - arbs + HA.

Faris are supposed to be a defensive unit in desert battles, if you ask me. Get invaded by armored catholics and run them tired with the Faris. I wouldn't use them otherwise, I think, since there are other options for the medium cavalry role.