enma was just a plot device to highlight zoro's potential.
everyone knows that haki is levels, different techniques require different haki output percentage:
-hardening/black = 20%+ haki output
-invisible = less than 20% haki output
-enma = 100% haki output. (pre-1033 zoro was regulating it to around 50-70%)
different people can output different haki percentages considering their levels of endurance:
-sanji for example can't even use hardening, if he tried to use enma he will literally be reduced to a dried up husk.
-luffy uses a lot of haki during G4 and that puts a lot of stress on his body.
-zoro barely could handle enma's 100% effect and his arm got sucked dry in a single swing.
zoro/enma dynamic:
-enma forces zoro to release all of his haki.
-zoro normally only uses enough haki to make his swords black.
-zoro can go all out with haki any time he want, the only problem was that zoro mistakenly thought that it might kill him, judging by how things turned out the first time he used enma, zoro had to train in order to raise his endurance to manage greater haki output.
that's why chapter 1033 made clear that enma was neither a blessing nor a curse. but in fact it was zoro who was holding himself back by not fighting with his full strength.
the moment that zoro got over his self made prohibitions, and started going all out, enma become like the rest of zoro's swords.