the breadcrumb dropdown selectors do appear at the top of category pages in mobile
and on mobile discovery (/latest, etc) pages in some themes, such as FKB Social.
You mention that you removed them because “the categories tab is right there” - assuming you mean the nav pill button, you have to click it to go the categories page then find the category you want to go to - it is much more intuitive to select from the category dropdown to go straight to where you want to go, especially if one is looking for a subcategory. Also, how does a user do tag navigation on your site now?