While I did focus my experiment on ThemeForest, the plugins I've built and those that I plan to build I think would be better for all theme authors and, by extension, users. If we can get some folks to adopt a few standards, it'd be great.

On the specific example of portfolios, I know I had mentioned the same issue to Brian Gardner at StudioPress last year because they were adding the post type to a theme. I don't know if that's changed since then.

So, this isn't necessarily a ThemeForest-specific problem. It's just more prevalent there.

It makes sense to adopt standard plugins from a theme development point of view. There's less code work. You can just focus on the markup and design. Your users get updates without having to update their theme. Users' content doesn't disappear from the admin when they switch themes.

The two best plugin examples to me are WooCommerce and bbPress. WooCommerce already does this with products. bbPress already does this with forums. How many themes do you see with built-in products or built-in forums? Theme authors have adopted these plugins as the standards to build from. We need to continue down the path that these plugins have paved.