Tim Bray lists a selection of email clients for you in case you want to be teleported seven years ago (cool, I’ll long then short the bubble and will become a gazillionaire!), when you needed separate applications to handle tasks that are obviously related to each other. Outlook is a market leader because it’s leaps and bounds more usable and feature-rich than the Notes client, its only real competitor. I wonder how software such as Eudora stacks up to manage contacts, tasks, surveys, custom forms, or shared calendaring. And how do you integrate RSS, spam filtering, or instant messaging? (Has anyone ever heard of a Eudora ISV?) How do you custom-code mini-portals, integrate workflow, or build reporting applications on top of all that collaborative data?
Outlook definitely has many shortcomings. For instance, the person who decided that Ctrl+F would be the shortcut for email forwarding instead of text searching should be spanked. Text wrapping is broken for no apparent reason. Some tasks require too much manual work (who decided you can’t flag several messages at once for follow-up?). More importantly, Outlook outside of the Exchange context is crippled — Outlook is philosophically not fully an Internet client (remember it didn’t even support POP3 in its first version), and if there are Exchange ASPs out there for small virtual teams, they’re extra good at remaining hidden from their prospects.
But Outlook is also quite good for many things that are simply impossible with its "competitors." Have you noticed that there’s no market for separate charting applications anymore, simply because charting naturally belongs in a spreadsheet? Likewise, Netware stuck too long to its file and print story and lost versus a more versatile, even if less stable and harder to administrate NT Server. People who claim that feature integration is bloat (or mistake a full collaborative client with email which is just a subset, not the whole), usually don’t know or care how to take advantage of said integration, and I’m afraid they don’t really care about how people work either.
Here’s a user scenario for you. Take a team of a dozen people in the same company. They need to find shared availability and reserve a meeting room and a video projector for a 2-hour slot. Half of these people spend 50% of their time outbounds with customers. Try this with a disciplined team used to shared calendaring in Outlook/Exchange, you’ll get a smooth and quick process and you won’t need to touch the phone or send dozens of emails back and forth. That’s real-world feedback from my time at MS 4/5 years ago, not demoware. Do the same only through email and count the wasted hours and cancelled meetings. People will end up setting up things on the phone ("No, I’m not here on Monday. How about Tuesday at 2PM? Hmm, let me check with Bill and Joe, they told me they were ok on Monday 11AM or 5PM, but I don’t know after that. I’ll call you back."). So much for your "no worse" email client.