"Spacers save bearings"
Definitely get bearing spacers if you plan sliding.
The sideways motions puts a strain on your bearings and causes them to jump forwards and back during a slide. Adding a spacer in between your bearings limits the amount of movement and thus the amount of strain on each bearing.
"Only one available"
this fills the need for 8mm cores. Usually the old Kryptos, Landyachtz Zombies, and some other ones. It's the only 8mm spacer available so it's a must, but still it warps because it's not steel. It's not that bad though because you probably won't be going too fast with wheels using the old Krypto cores like the Zombie Hawgs. Get these if you have a wheel with that type of core.