As someone who did have to "step up", I think I did a fine job, but I wasn't exactly thrilled about needing to do 2 jobs now: my old job and the job of the person out on leave. For no additional pay.
I absolutely believe parents should take leave, and while the exact amount may be up for debate, I think that, generally, paid leave in the US is woefully short. But I think it's really shitty to just pretend the extra burden on your colleagues doesn't exist when someone takes leave, or that they'll necessarily see it as a great "opportunity".