In order for visual editor to function smoothly on large articles or in a context of a high editing volume, there are two issues that need to be relieved : performance, as it can take a while to load visual editor on an article of large size, even when it was already loaded once, and edit conflicts, as they are quite regular on heavily edited articles, and it is necessary to use wikitext editor to resolve them, defeating the purpose of having a visual editor. This would be pretty much required in order to achieve a successful implementation on wikis like the English Wikipedia.
The possibility to save an edit without leaving visual editor (meaning there would be two save buttons, one staying on VE and one leaving VE) would help on both counts. Indeed, it can take a while and be a hassle to (1) save the edit (2) load the saved page (3) relaunch visual editor (4) load visual editor. This means users are dissuaded from saving often, which goes against the very common habit of making several small edits rather than a big one, and increases the risk of edit conflicts. If a user could simply (1) save the edit (2) update the visual editor window with the edit and non-conflicting intermediary edits if any, it would be much less of a hassle for the user and the loading time could be optimized by only reloading the updated elements.