Currently, the easiest way to manage multiple email accounts at the same time is by using a standalone IMAP client. Five accounts in the sidebar that I can access with a single click, with Unified Inbox at the top!
Gmail's web interface only lets you access one email account at a time. You could have delegated accounts, but they open in a separate tab, and the whole concept of delegation only works for Gmail accounts anyway. Or you could forward everything to your Gmail account and call it a day, but some employers might have a problem with that, especially in light of Google's apparently comfy relationship with No Such Agency.
Personally, I use my Gmail address to subscribe to public newsgroups, but I wouldn't let anything private ever touch a Google server. With a standalone IMAP client, it's very easy to maintain this kind of separation without having to suffer any noticeable inconvenience.
You might want a greater seperation of your inboxes, though, so your results may vary.
Also: because people might want security, such as GPG.
Downsides: Snooping, NSA honeypot, Chinese government hacker honeypot, etc., etc.
Here's one feature my email client has: automatic spell-check, such that emails are disallowed from being sent if they have any spelling errors (with an override, of course). Gmail has a manual-trigger spell-check, while the browser has automatic spell-checking, but neither have a personal (jargon-customized) dictionary built up over decades nor an automatic modal dialog if the message to be sent has a mistake.
I guess automatic spellcheck override dialogs are specialized, eh?
Or, more likely, if you're offline for any reason at all. All web tools disappear in a puff of smoke if there's a problem between you and your ISP. Or, more rarely, between your ISP and their peers.
https://chrome.google.com/webstore/detail/gmail-offline/ejid...
Anyway, I can't see why a tech savvy person wouldn't just have a domain and email hosting service instead of gmail and being locked to Google's whims, e.g. the ability to download could be a first step to shuttering gmail since gmail doesn't provide the core tracking data Google uses while creating specific privacy headaches - I.e. gmail is problematic to monetize efficiently.
Maintaining your own email server to the same level of reliability (backups, etc), speed, and functionality as gmail would be a lot of work. Not every "tech savvy" person out there wants to invest the time and resources needed to duplicate what gmail gives them for free.
[Also gmail's web interface is very, very, good, and it's well-integrated with other google products, which are very popular.]
I don't understand people who use the web interface. I guess many people are used to the general suckiness of web-apps.
However, I use POP3 and SMTP with a desktop client, not IMAP.