What DO Glaceon + Leafeon do for each other? They're used in different places, like Gardevoir and Gallade. Gallade is used to take out big opponent's and OHKO everything. Glaceon is used to slow down the oppoonent, like Gardevoir. Who knows, maybe in the future we'll see more Glaceon-centered Eeveelutions then Leafeon-centered ones, like G&G.
Leafeon's a BAM massive power house, who enables other pokemon while at it. Exactly. Like Gallade.
Glaceon locks the opponent. Like Gardevoir.
The problem is, the two need to be active in order to level up. Leafeon being active stops Glaceon's lock. Glaceon being active eats turns away from your "accelerated" set up. They have free Retreat, thanks to Umbreon. Now, they can easily Retreat, Level up, then BAM! Leafeon can hit hard, or Glaceon can lock.
They don't even cover each other's real weaknesses... :/ Leafeon won't snipe away Latios/Latias. How popular do you think Latilock will be? It takes up 2 Bench spaces, and a simple tech of Crawdaunt ex HP or Dusknoir.Glaceon won't stop things from locking powers, nor will it stop tricks to remove energies from the field. Again, how many of those are you expecting to see? Lugia is NEVER played, and ER2 is flippy.
Plus, you only get 4 eevees per deck. That evolution line is REDICULOUSLY crowded. I'd much rather play multiples of important cards then play a billion different cards, but only one of each, and even have a difficult time getting them on the field at the correct time. Well, it works for Gardevoir, it can work for Eevee. But, I agree, this is one of the few problems with Eeveelutions, is that you have only 4 Eevees, and you have 4 Eeveelutions you need to Evolve.
So yeah, the Eevee-lution list that works will be a tad tricky to see. So many different colors. So many different stage 1s off one basic. None of the Eevee-lutions have any sort of draw support, meaning you'll very much want to play at least one other line, which can throw off starts. You run Claydol in Eeveelutions. Problem solved.