It's no question that in RBY, Ivysaur/Venusaur is the weakest starter in mid/late game. Ivysaur's only real merit over the other two (and what puts it at the highest rank in in-game tiers) is that it is the simplest and most consistent way of beating Misty.
It's THAT big of a deal to beat Misty, arguably the relative strongest NPC in the game (she is-- you don't have to agree, but if you don't even see the rationale for thinking so you seriously need to play some more, without Ivysaur). It's a testament that Starmie even destroys Yellow Pikachu (unless you get really lucky with X-Defends).
"Hmmm... a name for this plan..."