As W6 said. Also, another purpose of jump rope for the boxer is to work on keeping your feet light and your shoulders loose, relaxed. Has worked tremendously for me.
And it does take time to build up to that 3-minute round. Oh yeah.
As for steady/long term cardio. I still believe there is a purpose. It's good for someone who is embarking on the road to "get in shape" to begin with long term cardio. You know, to build up lung capacity. Once there however, then begin HIIT.
I realized this after having ran in the Hood To Coast back in '98. This is a 200-mile relay that is, yes from Mt. Hood to the coast. 2-teams of 6-runners each for each group. 30-hours of NO SLEEP. Wild. I trained for maybe 3 to 4-weeks out. I ended up running 8-minute miles (6.5; 7.2; 4.8 mile legs). And I ain't genetically suited to running long distances, but this certainly makes me think when someone tells me, I'm in excellent shape, I can do cardio FOREVER...."
Uh huh, more than likely they HAVE NOT tried HIIT. Kicks my ass everytime.