Secondary skills in Heroes 3.

That's exactly what I wanted to find out; in my opinion, the game isn't worth the effort. Wasting a slot just to possibly gain one level is too much of a waste.

Absolutely correct. Only sometimes, specifically one level is missing to complete a quest, and you have to replay many turns, take this skill instead of another more important one during the game, grind the hero for XP, and potentially break through the quest a week earlier than your opponent. And that is already a strategic advantage on the map. So one cannot make categorical statements about usefulness/uselessness... We are talking about offline.

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And you won't even gain a level—it's a diminishing progression...

What is meant is—reaching the next level sooner...