Google basically states that the mixed case should make little to no difference to them, but they prefer consistency in the URLs, and in these terms the mixed case is the worst possible choice there is.
I agree with you regarding your statement that the mixed case has no SEO impact when it comes to eZ routes. It doesn’t really matter which case you use, since eZ will 301 redirect you to the version of the URL with the capitalisation that is stored in the database.
However, the bigger problem is trying to explain this to the client or the SEO teams. Most of the SEO teams have little to no experience with the eZ, and usually are blindly following the same guidelines for eZ websites as they would for some Wordpress site.
Most of the clients insist on this because they are told this is the best practice and they want to follow it no matter what you say. Which kind of becomes a huge problem when they decide to do it after two or three years of site being in production, and we have to regenerate URLs for 100+ thousands of locations on a live database. Not to mention potential loss of the history alias entries during the process, and the number of redirects that will occur after the conversion to lowercase - and that will eventually make a significant impact in search indexes.
If you would state that the SEO should always have been done before even going live, I would absolutely agree with you. Unfortunately, we don’t live in the perfect world, and most clients tend to ignore it or won’t cover it with the project budget before going live, and they will realise that it actually makes a difference only after they see the drop in page views or search index ratings.
So to summarise my point, as a web developer, I would rather try to avoid the problems above if possible Nobody ever wanted to switch the lowercase to mixed case, it’s always the other way around.