I actually think it IS an appearance thing. I’m not sure what you mean by the “average client” but I assume you mean content editors.
Sure, those may you care about posts and pages. But of course there’s more to taking care of a web site than just content. I see a client as someone who takes care of most aspects of a website after I’m done developing it, for example. If they want to change items in the footer, or if they add a page/section and it needs to be added to the menu, or if they want to use a different theme, then these are all things that deal with how the web site looks — hence, that’s why they’re all in the Appearance tab; things that appear on the web site.
And the client may definitely want to move things around in a menu, so it’s logical enough to me.
Having said that, if you want to move things around in the menu, there’s plenty of plugins for that. I don’t use that myself so I can’t give you a recommendation, but a quick search should give you a lot of results.
Another “Nice to have” would be to have the “Menu” administrator visible to Editors by default.
I guess that’s why there’s various user roles. Some can just do content, others can do content and appearance, etc. But I guess that would take another plugin.