The limit with outlook express is just over 1300 chars and I assume that is the same for other clients. Have not get the facilities to test with many other email clients, so I truncate to 1000 chars.
Why bother with the middle man of the email client software?
Depends on the application. My system allows our clients to monitor our work for them, online-reporting. If they want to email us about a case then the link pops up their client and inserts the ref and the paticular point in question. Using their email client to send us emails means that we get the correct reply to address and they get a record of the email in their sent items.
Forgot the main other thing, it allows them to add whatever they want to to the mesage, including attachments.