Interesting, and I agree there are benefits of webmail.
However, there are quite a few disadvantages Bob Rankin did not mention.
Firstly, the email notification. You don't get the mail notification on your email icon. You will have to have a browser window to keep checking for new emails. Something like the “2” badge over the mail icon is a convenient feature.
Composing and attachement. For adding a picture into my message with Mail.app, I can simply drag the photo into the composing window. On the webmail's composing window, I would have to click on the attachement icon, browsing through my computer, and find the photo.
Also, there are times that I would like t send some executables as attachement.
Next, the advertisement. I do not see the advertisement on the webmail a big issue myself, however, there are many extremists would see any "sponsorship" as pure evil. To best of my knowledge, except Apple's (paid) MobileMe email service, all other major webmails do have advertisement associated.
Also, offline reading. With my own email client, I can download all my emails, and read them (as well as composing the replies) when I am not connected. With webmail, you will need to read while having internet connection.
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