I feel like I'm clogging this thread, but I gotta get all my arguments in before I leave on vacation! Sorry!
1.)You can make more droids. (See thousands of worlds allied)
You can't make more Mandos. (See elitest nature and need of lifetime of training.... and being born.)
2.) When end-game comes, I have the feeling Grievous and Assajj and Magma Guards will team up to take on Mandalore. He's probably either
a. Too honorable to call for help
b. The only survivor/ no one -can- help him.
He won't be a match for the two of them and the droids. Beskar might be immune to lightsabers, but not the Force. Or zap-rod things.
3.) I doubt Cassus Fett can do anything for Mandalore other than command the army. Grievous is already a master stragegist, so it's not like it's tactics vs. none.