All three fully evolved Rock/Steel types can learn Iron Defense, all three have high base Defense stats (Aggron: 180, Bastiodon: 168, Probopass: 145), the latter two mentioned in the parentheses have high Sp. Def., and the two types that deal 4x damage are mostly offensive. In Emerald my Aggron took an Earthquake from a Slaking while at +2 Defense and didn't even lose half its HP. It may be a bad combo, but it has yet to be given to a highly offensive Pokemon.
Grass/Psychic and Grass/Ice, on the other hand, have seven weaknesses, one of those seven on each other's part being a 4x weakness (Bug for Grass/Psychic and Fire for Grass/Ice). However, Celebi has decent Defense and Sp. Def. and both it and the Egg line (Exeggcute and Exeggcutor) have more resistances than Grass/Ice, so they're better off than Snover and Abomasnow.