On Exchange, that's hardly the case. Just the act of migrating a relatively small mailstore from old to new can take hours and hours. One recent Exchange migration had the GUI progress meter for the Move Mailbox operation run to 100 percent for the mail migration within 5 minutes, which was puzzling. A few checks showed that the migration was nowhere near 100 percent, but was still running. The progress meter then proceeded to have an epileptic fit of blocking and clock face jitters for the better part of six hours before the 15GB of mail was finally moved. For those scoring at home, that's roughly 4.5Mbit, or 600KBps -- and this was between two dual-CPU servers with gigabit interfaces and six-spindle RAID5 arrays.
If there's one thing I can't stand when doing dicey migrations, it's the fact that these long, overly onerous processes are a one-way trip. If at the end of that six hours there were problems with the migration, the only backout plan is to try to roll back to the original server, since the maintenance window is shot. Also, I intensely dislike it when progress bars lie to me right out of the gate. Limbo is not a comfortable place for an admin doing an overnight migration.
It's also not lost on me that many Exchange implementations don't deal directly with the Internet -- there's generally a mail relay filtering e-mail before it ever gets to Exchange. Personally, I would never install Exchange as a front-end MTA. I prefer using a Linux prophylactic running Sendmail, MIMEDefang, and a virus scanner.
But without Exchange, we wouldn't have Outlook, and without Outlook, I imagine people would simply wither at their desks, completely confused as to where their meeting was located and who was in it. It's true that most organizations need the features provided by Exchange, and while there are open source products that provide those features, they're not as streamlined and integrated as the Exchange/Outlook combination. I've often thought that if a company wanted to make serious inroads on Microsoft's grasp of the business world, that's where to start.
If you build a better mailserver, they will come. Maybe that's why the grandaddy of all mailservers is still the most widely used.