When it comes to the pokemon games, almost every kind of evolution benefits in terms of power. Only a few uneducated Pokémon have broken this stigma with the Defense-boosting Eviolite or advanced strategy. In addition, the benefits of evolving a Pokémon for battle generally outweigh any losses.
However, as usual, the anime tells a very different story than the games. To quote Ash Ketchum from Pallet Town: “It takes a lot more than just evolving to get strong (Black and white Episode 10, A Rival Battle for Club Champ!).” In addition, there are all kinds of reasons why a Pokemon doesn’t evolve or won’t evolve in the anime. In the grand scheme of things, evolution doesn’t carry the same weight in the anime as it does in the games.
It’s worth noting that fights in the anime also don’t have the same competitive values as the games. There is no such thing as optimizing nature and EVs, nor does it seem to matter whether a Pokémon specializes in physical or special attacks. Instead, battles focus on which Trainer can better coordinate their Pokémon and make sure they fight at their best. In that sense, it’s less about the Pokémon being used and more about the Trainer using them.
Because of this difference in how battles work in the anime, undeveloped Pokémon can defeat fully evolved Pokémon all the time. A Pokémon that’s fully evolved certainly makes them harder to defeat, but it’s nothing an experienced trainer with an unevolved Pokémon can’t overcome. Whatever difference in power evolution makes, it’s not nearly as absolute as in the games, though it is doing make the undeveloped Pokémon the underdog.
It is especially worth noting all the Pokémon that defeat their own evolved forms in battle. Ash alone pitted his Pikachu against Raichu, losing and then winning in a rematch at least a few times in the series. He has also used his Snivy to defeat a Servine, his Rockruff to defeat a Lycanroc, his Rowlett to defeat a Decidueye, and his Torracat to defeat an Incineroar. His Tranquil also defeated an Unfezant, but only after evolving into an Unfezant himself. According to the logic of the anime, each Pokémon has the potential to compete at the same level as their own evolution, if not better.
There are also some potential negative consequences that can result from a Pokémon evolving. For example, some Pokémon can only learn certain moves by leveling up in their pre-evolved form. This was most famously illustrated during the rematch between Ash’s Pikachu and Lt. Surge’s Raichu. Because Raichu evolved too quickly, it couldn’t learn any of the quick moves that Pikachu could such as agility and quick attack; this indicates how many brick-evolving Pokémon miss learning moves due to leveling up if they have evolved too quickly. This is a point against rushing to evolve Pokémon for both games and the animation.
Pokémon that don’t evolve can also benefit by learning moves faster than their evolutions would. In episode 51, “Bulbasaur’s Mysterious Garden”, Ash’s Bulbasaur was able to learn and use Solar Beam to defeat Team Rocket. If it had chosen to evolve into an Ivysaur like the rest of the Bulbasaur in the episode, it probably wouldn’t have gotten this edge.
A Pokémon that is not ready for evolution may also experience other disadvantages. When Ash’s Turtwig evolved into Grotle, his new mate made him less agile, which he wasn’t sure how to handle at first. Apparently, such a change in a Pokémon’s biology like evolution can really mess with how that Pokémon fights. As an aside, Ash’s Torterra never won a big fight after its development.
Since evolution isn’t necessary to make a Pokémon strong, many trainers leave it up to their Pokémon whether they want to evolve. The decision is difficult, because a Pokémon that evolves cannot go back. Making one’s own body undergo such a dramatic metamorphosis has several implications, not all of which have to do with fighting.
Perhaps the most famous example of an anime Pokémon that won’t evolve is Pikachu. The topic came up in Eoisode 14, “Electric Shock Showdown”. After losing to the very hostile Lt. Surge and his Raichu, Pikachu was offered a Thunder Stone so it could evolve; according to Surge, evolving Pokémon right away is the ideal strategy to make them strong. However, both Ash and Pikachu realized that if the latter evolved only to fight, it would prove that Lt. Surge was right. To hold on to his own beliefs and values, Pikachu was determined to defeat Raichu as he was.
Dawn’s Piplup also didn’t like the idea of evolving into a Prinplup. It even used Bide to expend all the energy that would have helped it evolve. Apparently, Piplup wanted to stay as it was the day he met Dawn to preserve the memory. To that end, Dawn gave Piplup an Everstone so it could prevent its own evolution without passing out with Bide.
Another Pokémon that prevents its own evolution with an Everstone is Ash’s Rowlett, although this has nothing to do with not wanting to evolve. In Sun and moon Episode 97, “No Stone Unturned!”, the Grass Quill Pokémon found the stone in the forest, liked it, and promptly swallowed it. Despite Ash’s attempts to get the rock out, Rowlet quickly grabbed the rock and swallowed it again. The only advantage this gives is the ability to use Seed Bomb by spitting the stone at his opponents. It probably doesn’t realize and doesn’t care that the stone is hindering its development.
All this, of course, is not to say that evolution is inherently a bad thing. When a Pokemon is doing evolve, it is treated as a momentous occasion that will hopefully lead to bright new horizons. But again, with the way battles are played out in the anime, there’s no real rush to evolve; it’s just something Trainers can feel free to do at their own pace. When a Pokémon is ready and able to evolve, they will often be happy to do so.
The main conclusion is that evolution is not the ultimate it is in the games. Whether a Pokémon in the anime takes the time to develop or just doesn’t want to, it has nothing to worry about. Likewise, if a Trainer prefers to keep his Pokémon cute and cuddly, he can do so without fear of missing out. As long as the Pokémon is happy and the Trainer knows what they’re doing, that’s all that really matters.