If you want to trim the content, you can always use more tag to control how much contents for each post to show on archive page.
The "more" tag is not the same as a custom excerpt. Custom excerpts allow the blogger to write exactly what he or she wants to use as a summary, including choosing a smaller picture which can be aligned differently from arbitrarily truncating a post.
The reasons for this change is that we've received a fair amount of requests from users who want to display posts on archive pages in the same way as on posts page, and we believe this is better for consistency.
I've not seen folks asking for such a change in these forums. With the custom excerpt feature it was/is possible to copy the whole content of a post (text editor) to the excerpt module so if someone wanted whole posts in categories and archives they could have them. (except for embeds and shortcode).
IMO making this change allows for less blogger control of what is published. Is it possible to make custom excerpts a feature we could opt in to use?