Poison doesn't really have a purpose right now, since you don't see Fairy types in gyms. They were already uncommon to begin with but the recent CP update furthered it. Wigglytuff got a relatively large ~14% CP nerf, and while Clefable's reduction wasn't as large, it's more that other Pokemon also got buffed so Clefable is still worse off relative to other defenders.
Saying Poison has a use against Fairy is like saying Aerodactyl with Steel Wing / Iron Head also has a use too - you'd probably be better off with a good Pokemon that does neutral damage with STAB instead.
Like you said, the top tier grasses are either Poison which cancels out the super effectiveness of your Poison moves, or in the case of Exeggutor, it's Psychic moves deal extra damage to you, so you're still on even footing.
On a side note, Parasect is actually a decent attacker. I took down a ~2300 Vaporeon with a 867 CP Bug Bite / Solar Beam Paras just now, with 6/77 HP to spare, since it had Hydro Pump. If it had Water Pulse or Aqua Tail I probably would have lost since I'd take more damage during Solar Beam.
On paper, Bug Bite has similar DPS to the typical 6 damage, 0.5s duration moves, but in practice it can pull ahead due to the damage formula giving +1 to all attacks, meaning faster moves gain more benefit because you use them more often.
