Manasteal has been tried waayyyyyy back in the beginning of league and was decided to be extremely unhealthy.
Items that build from Tear are for champions with large mana pools that spam skills consistently AND BUILD MORE MANA ITEMS. You have to realize, certain builds will give you the equivalent of a free BF Sword or two on certain champions because the AD scales off your max mana. When you're playing a champion that benefits from these playstyles, you're typically casting a lot of spells, which charges the item REALLY FAST. Ever seen an Urgot? His Q is basically an auto attack xD He charges up tear faster than Rammus' Powerball.
The item is a niche item like Guinsoo's Rageblade, Nashor's Tooth, Wit's End, etc. and is only meant for champions who build certain ways.
Items built by people this item synergizes with: Iceborn Gauntlet, Frozen Heart, RoA, Lichbane, Righteous Glory, Trinity Force, and Zeke's harbringer
Muramasa users (complete list, only ad champs) include:
URGOT, YORICK, EZREAL, Corki, Sometimes Alistar, Sometimes Evelynn, Jayce, Occasionally Kassadin, Kog'Maw, Occasionally Taric, Occasionally Udyr (Bludyr is actually pretty insane, only against full ad comps)
Seraph's Embrace Users Include:
Swain, Singed, Occasionally Sona, RYZE, Sometimes Nunu, Nidalee, Lissandra (Try it lol), Kog'Maw, Kassadin, Karthus, Evelynn, Ezreal, Cassiopeia, Corki, Blitzcrank, Sometimes Brand, Alistar
Reiterating: The item is a NICHE item and is meant to synergize with other items, or the champions kit. It's actually a super powerful item, don't believe me? I dueled a fed Nasus with 200+ Stacks as Urgot and won. 1v1. Neither of us moved. All I did was cast spells repeatedly and auto attack. He was full Armored out. How? Blurgot build 8D Muramasa, Gauntlet, Heart, BC