1. When you get to packing 10kb binaries by themselves, the advantage in separation tends to disappear. There is some overhead per loop mount, generally unnoticable, but if one started to mount many 4kb .tcz extensions the overhead might be more than the extensions themselves.
There's lost compression (= more disk usage, having them together should compress better than separated), and I'd wager a guess that the gained boot time from dropping 4kb from the initrd is less than the time it takes to mount one 4kb extension (a net loss, that is).
2. It is faster to boot than MC + extensions. It's good to keep choice and a fast X build, and its creation from MC + the extensions is automated.
It's also "just two files", one point that may be appreciated by some.