Ah, email programs. From a technology standpoint, they are downright horrible. Here are a couple reasons to avoid Outlook. Though Apple Mail, and others usually fall into the category.

1. Instability - In my experience, and after answering literally thousands of outlook or apple mail based support requests, I've concluded that the programs are just purely unstable. The more mail you have, the worse they get. Between randomly failing to get mail, send mail, or erroring out, they become more of a headache than they're worth.
2. Cost - Time is money, and the amount of time that gets spent troubleshooting a faulty program can be extensive. Money can be saved, as well as time by just using web-mail. Outlook problems are one of the most common daily requests in many help desks.
3. Security - Your mail is out there on your computer. If someone steals your computer, or gains access to it, it's not as simple as changing a password to lock someone out. The mail is available for the taking. With Webmail, this isn't the case.
4. Data Loss - There's no guarantee that all your data will actually sync, meaning that if the program dies out, glitches, or technology happens, you could lose data. I've seen this before with someone using outlook. Calendar entries got put into a black hole in outlook and disappeared, not recoverable.
5. Efficiency and Integrations - We all want to become as efficient as possible. There are little add ons here an there that can help. However, most of the time, these add ons make outlook slower and less stable. Which kills efficiency. There's also the fact that things can't run in the background while your computer is off with outlook the way they can with webmail. Such as certain types of rules.
6. You don't need it - Become a master with webmail and you'll realize there are better alternatives to Outlook that will help you get what you need done.