This whole "A hero is only as good as his villain" idea has really got to stop. It's been used to justify too much laziness.
I don't understand that statement at all. It's the other way around. Laziness is making a villain one-dimensional because you've put all your work into the hero. Like I said, you want to put as much care as you can into every aspect of a story. It's all part of the same whole, and lack of quality in any portion of it undermines the rest. How well will a race car run if you put all your work into giving it a fancy custom engine but slap on cheap, inferior tires and shock absorbers? How good will your lasagna be if you make it from gourmet pasta and tomatoes but use low-grade store-brand cheese?
But look at most Batman movies, they're really just about the villains with Batman reduced to "Guy who is purely reactive". Why can't heroes ever be the proactive ones seeking to change the status quo? And no, I don't mean "Clean up Gotham" or something like that.
Most Batman movies have, frankly, not held a candle to the comics they're based on. They're a really, really poor benchmark. Aside from
The Dark Knight, I wouldn't count any of the live-action Batman movies to be among the best of the superhero genre.
Look at
The Incredibles. The focus there was on the family of heroes, but both Syndrome and Mirage were well-drawn characters with understandable motivations. Look at how many of the X-Men movies are a double act between Xavier and Magneto, the complex friendship and rivalry between two equally well-drawn characters being the driving dynamic of the story.
And that's the key. If a story is driven by the tension between hero and villain, then it hurts the story if
either one is underdeveloped. The Batman films suffered from an underdeveloped hero, and many MCU films suffer from underdeveloped villains.
If they have a limited amount of time, it's better spent on the hero who will be sticking around and his supporting cast. Internal conflict can make up for a villain who doesn't steal the show.
They don't have a limited amount of time. Movies take years to make. They go through dozens of script drafts. There's zero excuse for not taking the time to give every character dimension and substance. And if you're talking about the amount of screen time each character gets, that's irrelevant. As I said, a good storyteller can give a character humanity and depth in a single scene. The amount of time something is onscreen is unrelated to the amount of time a writer or director or actor can devote to creating it, to making even a brief appearance interesting.