It makes the whole format counter-intuitive / work against itself: Surely the point of the ‘all apps’ list is for the user to find an app that they don’t use very often (so therefore icons + labels are far more helpful), yet the apps that the user has pinned to the Start menu has labels (even though the user is a lot more likely to know the icon art for their pinned items).
Furthermore, I like to think that I know every app that comes with Windows, but rather like me occasionally deciding to deep-clean my keyboard and removing every key in order to clean all the nooks and crannies, and I’d like to think that I know where they all go without thinking, I take a picture of the intact keyboard first because I know from experience that I’m not as familiar as I’d like to think (and yes I do touch-type), and that’s just aside from the fact that Microsoft has been helpfully “updating” icon art lately. Yes, one can hover the pointer over each icon for a second to get a tooltip, but imagine how much longer that would take to find the app one is looking for vs. having labels! After ten hovers I’d bet the user would give up and find some other way.