You want to compare it to Julianos? In that case, Rattlecage has 59 extra SD. Julianos has a max magicka component, which is worth 92 SD, before any amps from CP spent, racials, passives, etc. So in terms of the raw stats for base damage, Rattlecage is already behind.
But let's say you actually want that health, so let's leave the health-vs.-mag bonus off the table. Julianos also adds 6.2% crit chance. In PvE, that's about 4-6% extra DPS, depending on your build. Crit chance is less useful in PvP, but even against an opponent wearing all-impen, that 6.2% crit chance translates into about 2-4% extra DPS, depending on your build. To put that into perspective, 59 extra SD is only worth about 1% extra DPS on a typical endgame magblade build.
Really, the only reason to even consider Rattlecage is if you really, really want to save on potions or a skill slot for Major Sorcery. Outside of that relatively niche scenario, Rattlecage makes no sense.
I've seen someone trying to sell a bound rattlecage lightning staff called Despair
Rattlecage as a heavy armor set really pisses me off as it would be ideal for my sorcerer...