Originally Posted by Fat Skorpius47
Serperior is still better than most other Grass-types you can get, especially Lilligant. See, Petilil requires an Evolutionary Stone that isn't directly handed to you before your third Gym Badge and then will need a Heart Scale if you level it beyond Lv28. And for what? A Pokemon with less overall stats than the Starter and less coverage. Despite Snivy's Coil coming a little later, it's at the same level it evolves into the final stage (Lv36 on Servine) and is just as helpful given the role Serperior will play (Defensive and supportive). During my Snivy run, I was very close to replacing the sucker but I stuck with him and it paid off.
An NPC gives you the Sun Stone in Nimbasa, and you shouldn't be using Grass Pokemon against the third Gym anyway unless you like being roflstomped.