These are necessary if You're going to upgrade to brushless power! I've tried both the stock differentials and ball diffs. I think the original differentials are great and they can, except for these, take a lot of punishment!
I suspect they will get worn a bit quicker then the ones in steel for ball diffs, but it'll still take quite some time.
Thay could've been a bit cheaper, but they're still alot cheaper then ball differentials.
If You want smooth diffs and a car that can take all that brushless power, this is the way to go!