Skeletons are kinda shit, but a large reason of that is how fucking slow they are and how squishy they are, so half the time you summon them directly in melee range of stuff and they die before they get a hit off or stuff moves out of the way and they don't hit shit, it's a pain. Mages on the other hand don't have either of these issues, you summon them far from the enemies since they're ranged and they don't have to move to attack. And the damage output of skeletons isn't too bad and was buffed in 2.0, they removed the low damage efficiency on them iirc so they fully benefit from auras and shit.
You would need 2 of these jewels I think so you can have 6mages, and obviously you'd still have your zombies and probably spectres for this type of build, but now you have a new kinda interesting skill you can use after you're done cursing. Can link it on spectre links since it's likely they'll be using projectiles so they'll benefit from similar supports as spectres. Note that spectres themselves were buffed a bit(and have a bigger buff coming once they unlock Necromancer but that's in a while).
I don't think it'll be super good, but it's interesting for spectre summoners and might be worth using because why not. Also we don't really know what skele mages are in the end, maybe they're just the normal skeletons like in Vaal pyramid, or maybe they're a lot better. They gave zombies a proc that makes them do aoe. Maybe they gave skele mage a proc too, or maybe skele mages have normal move speed and last longer so they act more like normal summons. Hell maybe they don't even despawn, think about that, they could just be like zombies/spectres and be permanent.
Mostly it's exciting because summoners don't really get much new stuff very often. They added golems with Awakening but they're super boring and everyone uses them too so it doesn't feel like a summoner thing, they're not that powerful even in a summoner build and shit. The uniques rarely change summoner stuff either, they just make existing shit better, like the staff they're also adding that buffs your minions based on the minions you have, it's neat but realistically it doesn't change shit. So any change is welcomed, and a lot of people play summoners if only to farm currency at the start of new leagues.