I think Crowe's accent adds gravitas to his lines, but that's possibly because its coming off as more of a UK accent to me. I'm no expert at most accents, though.
I think it's better that both actors kept their natural accents. It allows them to simply focus on being sincere in their performances and actually adds some interesting subtext to their delivery, especially Shannon. His Zod's about as aristocratic as a can of oysters, so he really comes off as a focused, down to earth soldier who's fed up with his leaders failures. I love the change in dynamic. He's easier to empathize with, but also scarier in that we can see how he grows nihilistic and can't change from his binary coerce/attack method.