Depends, it's the computer in the common room that's never locked and it would be a pain in the ass to have to switch OS profile every time one of us wants to check something on the internet that requires access to our personal accounts and bookmarks. The web browser is the only thing that needs to be separated. I don't think it's a particularly rare situation that people in the same family share a computer but want separate browser profiles so it's in sync with their cellphone.

With Chrome we each have an icon in the taskbar and our instances are merged under our respective icon in the taskbar.

With Firefox "vanilla edition" we either need to access about:profile to switch or we can use a workaround to have separate icons in the taskbar but then because it's not Firefox's regular icon that we're using but a shortcut modified to open a specific profile, each shortcut creates a new icon in the taskbar when we click on it and that's where the instances merge, that means that if we both have instances opened we now have four Firefox icons in the taskbar and because of muscle memory we tend to click on the shortcut instead of the icon where our instance actually is so we end up opening a new instance instead of just opening the instance where we already have our tabs opened.