It can be a pain removing stuff from the rack to fold or half fold a bike. From experience though other systems have their down sides. Ie a Brompton is difficult to carry front bag fully loaded and you are best removing it for steps anyway. I found the quick release rack bag on my mezzo just as convient.
Also a front bag makes the bike unstable half folded.
As long as the rack bag is quick releae and has a shoulder strap you are missing out on many refinements to convience in reality.
Ps its possible to fit the Mezzo rack system to other bikes, using adapters. See the Mezzo upgrading thread.