Theme components also add features to Discourse forums, so disabling them by default can also result in a worse experience. Placeholder Forms is one example used here on Meta.
A topic is easier to understand if you see what the author wanted you to see:
Instead of just this:
This is one example of what changes when you apply safe mode on all forums (which includes Meta). Other forums may have different customizations they expect their users to see. Banners with information, previews of hidden categories, etc.