I have a lot of background processes. Why would I run another one? Why would I use my browser via localhost when I have a GUI client.
I want to read my email when I read my email, and otherwise don't want my email client it to take up system resources. If I want to be notified when a new message comes in, I'll keep the application open.
Plus, I have so much stuff in my system tray, the last thing I need is another sys tray icon.
Applications that install services are perhaps my absolute pet peeve. I refuse to use iTunes to this day for just that reason.
Memory usage for the Zimbra service is VERY high. Consider this, with the GUI client *off* (so just the background/service process running)
firefox: 38K (less usually).
So I can run 12 mysql servers for the cost of the Zimbra email service. Crazy.