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Decided to take a look at some other skill activation values just to see. Here are all Ingredient Magnet pokemon for example.
Main thing is that Slaking gets an activation rate boost that effectively matches its output with the skill-types. Though by evolving, it severely hampers its actual berry gathering power due to its helping speed decrease. So depending on the skillset, it may be worth keeping a Vigoroth instead of evolving it, especially if you're looking for berry power.