In my experience, global inbox is not just useless, is something that makes messy a mail client (and it’s the first thing I turn off, if possible, on webmail).
You are obviously free not to use it. To each his/her own workflow.
But it’s a requirement for many users these days.
We all have a crap email (for newsletters, commercial registrations, online shopping, dumb stuff, etc…), usually Outlook, and a serious one (for friends, more private stuff, to apply for a job…), usually Gmail. Not 100% accurate of course, but I’ve seen the pattern with most people I know.
And you can’t call it messy. In Evolution it might be, since it’s a horrible hack to get a global inbox-like thing.
But in Thunderbird it’s absolutely neat. Completely integrated. You can even add a recipient or account column to make the distinction very visible. And you can still select each account separately. You can also pick which account to send the reply or emails from. It’s anything but messy.
First thing I activate. I don’t even consider the clients not having the feature. But when Thunderbird was said to be abandoned a couple of years ago I tried a few different alternatives and dismissed some quickly because the feature was missing (Geary). I also use Bluemail on Android because it does unified folders really well too, with different colors per account.
IMHO the only Evolution’s problem is that it doesn’t fit at its best on low resolution computers (like netbooks, for example).
Yes, maybe its appearance is a bit outdated but it allows you having both imap and bop accounts together, setting up rules and filters, letting you natively signing and encrypting mail via GPG or S/MIME, configuring DS contacts, …; and it has a nice calendar that can be integrated with BigG and/or MS calendars to have synced your pc and your mobile.
I said Evolution isn’t too bad. In my way of saying things, it means it’s good.
I just explained why I won’t use it and some limits people need to be aware of. But indeed if one needs a more professional Outlook substitute (with Exchange calendar integration), it’s the way to go. Especially for the calendar. Which is why it’s more of a pro mail client in my opinion. For most users, it’s like using Gimp to look at holiday pictures and crop them occasionally.
And it’s highlighted by your GPG mention, beside a few security paranoids, not many common users outside of corporations will set up GPG for personal emails.