1. Magic Bounce Pokémon completely wall Lugia. This is exponentially more important than before now that Multiscale Lugia is released. I'm sure many people here have experienced having their team destroyed by one of those Toxic-stalling Substitute Lugia, and I'm sure that in that situation, many of such people would probably at some point have wished that their team contained an Espeon or a Xatu.
2. The inclusion of an Espeon or a Xatu in one's team allows one to completely counter the vast majority of offensive threats with Multiscale Lugia, as I have explained before.
3. Ho-Oh is not the only Pokémon that dislikes being damaged by Stealth Rock. The aforementioned Multiscale Lugia, Reshiram, the two new Kyurem Formes, and Rayquaza all despise it. Actually, technically, every single Pokémon barring those with Magic Guard do. Regardless of how resilient to Steath Rock and other entry hazards one's team may be, being able to keep one's own field hazard-free is a massive advantage for any team, and certainly worthy of dedicating one team spot for an Espeon/Xatu.
4. Toxic Spikes put a limit on the life of one's Kyogre or Groudon, putting one at an immediate disadvantage in the weather war, the victory of which is extremely important to many teams.
5. Toxic Spikes render most Calm Mind Arceus almost completely unusable unless they have Refresh.
6. Ferrothorn Spikes stacking is one of the deadliest strategies in Übers due to this Pokémon's great bulk and myriad of resistances, which gives it ample time to eventually stack three layers of Spikes, at which point it becomes exceedingly difficult for the opponent to win especially if the Ferrothorn trainer has a bulky pseudo-hazer like Giratina or Lugia (it wasn't my intention to mention this Pokémon again...), as add in Stealth Rock, and most Pokémon are taking 30+% damage just by switching in. This is especially devastating against rain teams as they often struggle to defeat Ferrothorn quickly, but all of this can be easily solved by just sacrificing one team spot for Magic Bounce support.
7. Espeon with Yawn and Xatu with Haze are both perfect counters to Smeargle's Shell Smash + Baton Pass strategy.
8. Having a Pokémon completely immune to status effects in one's team is a huge asset.
9. Magic Bounce stops the Copycat Riolu strategy.