I've been including them from time to time as a cardio substitute. At a body weight of 250 lbs. I even find unloaded step-ups to be a fine zone 2 style cardio session.
However. Carrying a 32kg, or 40kg bell in the suitcase or rack position allows me to really push it. And if I get the rest periods right, I can purposely calibrate the session.
Lately, I've mostly been doing rack carries.
So, I like to use 1:2 work: rest ratios for the 40kg bell right now.
Many intervals have been useful. Especially 20, 30 and 40 second carries. Accompanied by 40, 60 or 80 seconds of rest.
And 1:1 work: rest ratios are a good fit for the 32kg bell.
Again 20, 30, and 40 second carries have worked well enough. And with the 32kg bell I've usually been able to make it through a 30 - 40 minute session with equivalent rests.
For me, my rack carries are good direct training for carrying my kids. And it's definitely increased performance. I can now carry my kids till they tell me to put them down.
One way to go about it A+A style...
Start out with 1:2 W:R, and keep working with it till you get up to 2:1 W:R , then pick up a heavier bell, and start at 1:2 W:R, again.
Vary the length of the carry to wave the load. From 15, 20, 30, 40 seconds . And wait till you're comfortable with 40 second carries before you start decreasing the rest.
Most recently I've ended up mixing my 32kg and 40kg Carry's and their durations and work rest ratios.
With a mix of work rest times and a couple different bells you can dial in a very good "cardio" session.