And you have to wait two turns, Bulbasaur is a special attacker, in Let's Go you can teach it Earth Power... Usefulness wise, Bulbasaur is infinitely better. And Venusaur is very bulky
It still helps Squirtle and Charmander, plus Defense isn't everything in a pokémon battle, so you can't just make Ivysaur be there and not attack, even if Ivysaur attack, it can't do much damage