Leafeon is in Partially Used for the following reasons:
- Excellent Attack (and Defense)
- Great Speed.
- Access to STAB Leaf Blade
- Access to reliable recovery in the form of Synthesis
Despite this, it is sill considered not good enough for NU, meaning it simply kind of becomes obsolete against them.
Thus, Leafeon is stuck in a place much like the Borderline, only, far, far lower.
Here's what Smogon has to say about it.
>Leafeon might just be one of the most interesting Pokemon around. With a great base 110 attack, good base 95 speed and a set-up move in the form of Swords Dance, it can act as a great physical attacker. Substitutes can absorb status efficiently if it doesn't run Heal Bell. Grass + Normal gives it very good neutral coverage, only missing out on a few Pokemon (Wormadam-T, Mawile, Drifblim, Shedinja, Klang, Bronzor, Lampent and Gastly). Alternatively, it can run Baton pass to pass its SD Boosts and/or Substitudes to it's allies, meaning that it can choose if wants to sweep itself, or support an ally. It also gets some useful utility options in Heal bell, Wish and Roar. This, along with its amazing base 130 defense and reliable recovery in Wish and/or Synthesis, gives it a useful niche as a defensive wall, with no unboosted physical attack in PU cleanly OHKOing it except for Rapidash's Flare Blitz. It can even act as a pretty good sun sweeper thanks to Chlorophyll. What places it in B-rank, however, is its atrocious base 65 HP and Special Defense stats, meaning that any neutral special attack will cripple it severely, or even OHKO it. It also faces a lot of competition from Vileplume, Tropius and Meganium as a defensive grass type, while simultaneously facing competition from Jumpluff as a offensive one. It also lacks Sleep Powder, which means that it will have a harder time setting up than, for instance, Jumpluff. All in all, Leafeon is still a big defensive and offensive threat that you shouldn't underestimate.
--Smogon on why Leafeon is B Rank in PU
EDIT: Yea, I screwed up. So, PU is below Nu, then. That minor detail got skipped in the process.
Basically, the NU got a boost, with Gen 6, and access to more legit HA + Certain moves. That caused our poor Leafeon to shift down a bit.
tl;dr: Leafeon is so bad, they won't even let it go to NU. It still has its own uses though.