Ludicolo: resists Grass, Poison, Flying, and Bug; weak to Ground and Steel (both bad offensive and defensive types, comparatively); without same type clause, loses resist to Grass, gains weakness to Water
Walrein/Lapras/Cloyster: resist Water, Electric, Grass, Fighting, Rock; no weaknesses; without same type clause, lose resist to Water, gain weakness to Water and Ice (except Lapras with Water Absorb, which only gets the Ice weakness)
Kingdra: resists Dragon, weak to Fire and Steel; without same type clause, gains weakness to Water
Torterra: immune to Electric; resists Fire, Grass, Ice, Flying, Bug; weak to Ground and Rock (bad offensive types); without same type clause, loses resist to Grass (common offensive type)
Tropius/Jumpluff: immune to Ground; resist Fire, Grass, Ice, Poison, Flying, Rock; weak to Water and Fighting (bad offensive types); without same type clause, loses resist to Grass, gains weakness to Grass (common offensive type)
EVERYTHING Ice-type: no weaknesses; resist Fire, Ice, Fighting, Rock, and Steel; without same type clause, loses resist to Ice, gains weakness to Ice (Regice isn't impossible to kill, yay!)
Rotom-Frost: immune to Ground (levitate); resists Fire, Electric, Ice, Fighting, Ground, Rock; weak to Flying (bad offensive type); without same type clause, loses resists to Electric and Ice, gains weaknesses to both
Mamoswine: immune to Electric; resists Fire, Water, Grass, Ice, Fighting, and Steel; only weak to Poison; without same type clause, loses 4x Ice resist (good offensive type, in general, without the same type clause, as only it can hit Ice super effectively)
Articuno: immune to Ground; resists Fire, Electric, Ice, Rock, Steel; weak to Grass and Bug; without same type clause, loses 4x Ice resist (see Mamoswine)
Jynx: no weaknesses; resists Fire, Ice, Psychic, Bug, Rock, Ghost, Dark, Steel (lolwtf resists everything); without same type clause, loses Ice and Psychic resists, gains Ice and Psychic weaknesses (phew)
Weavile: immune to Psychic; resists Fire, Ice, Fighting, Bug, Rock, Dark; only weak to Ghost (which sucks); without same type clause, loses Ice and Dark resists, gains Ice and Dark weaknesses
Venusaur: resists Fire, Ice, Poison, Flying, Psychic; weak to Water, Electric, Fighting; without same type clause, loses Poison resist; gains Grass weakness (important since the Grass/Poison combination it has as its STABs hit everything in the game neutrally except Sawsbuck)
Pure-Psychics: resist Psychic, Bug, Ghost, Dark; weak to Fighting (horrible attacking type); without same type clause, lose Psychic resist, gain Psychic weakness
Meloetta/Girafarig: immune to Ghost (Girafarig is also immune to Grass, if using Sap Sipper); resist Psychic, Bug, Dark; no weaknesses; without same type clause, loses Psychic resist, gains Psychic weakness
Gallade/Medicham: resist Flying, Psychic, Ghost; weak to Fighting, Rock; without same type clause, lose Psychic resist
Xatu/Swoobat/Sigilyph: immune to Ground; resist Electric, Ice, Psychic, Rock, Ghost, Dark; weak to Grass and Fighting; without same type clause, lose Psychic resist, gain Psychic weakness
Ghost-types: they're already weak to Poison and Bug, two of the best offensive types, so keeping them weak to themselves too is unnecessary; furthermore, it is hypocritical to switch Dragons but not them - Ghost-types should resist Ghost-types
Dark-types (Liepard, Umbreon, Absol, Zoroark): immune to Psychic; resist Fighting, Bug, Dark; only weak to Ghost; without same type clause, loses Dark resist, gains Dark weakness; also, if Dark can't hit Dark hard, it only hits Fighting and Steel hard, the former of which is bad offensively and defensively, and the latter of which is complete shit defensively anyways
Scrafty: immune to Psychic; resists Fighting, Flying; weak to Rock and Ghost; without same type clause, gains Dark weakness
Krookodile: immune to Electric and Psychic; resists Water, Grass, Ice, Fighting, Bug, Dark; weak to Poison, Rock, and Ghost; without same type clause, loses Dark resist, gains Dark weakness
Honchkrow/Mandibuzz: immune to Ground and Psychic; resist Electric, Ice, Rock, Dark; weak to Grass and Ghost; without same type clause, loses Dark resist, gains Dark weakness
Sableye/Spiritomb: immune to Normal, Fighting, and Psychic; resists Dark; weak to Poison and Ghost; without same type clause, loses Dark resist and Ghost weakness, gains no new weaknesses