This might be the first time I will give some props to Microsoft. It's not MS or Apple's fault, it's not a fault of anyones. As I mentioned earlier, and the last part of your linked article says the same thing, Outlook has this feature because it is generally used for intranets and closed networks which are generally enterprise atmospheres. Within a closed network it makes sense for you to get read receipt from your co-worker or boss.
Entourage is not made for enterprise, its just a standard email client like Apple's Mail.
Here's why it SHOULDN'T be included in standard email clients anyway:
Would you want your friends to keep tabs on you? Your emails should be very private. If a friend sends an email to you and you read it but didn't feel like responding right away but then later your friend asks you if you got his email you can then fib and say no, but I'll read it later.
With receipt request, it's an invasion of privacy. Everyone would know when you read their email and would ask you why you didn't bother responding? Because it's an invasion of privacy that's why all email clients don't offer this feature. Outlook is not designed as an email client, it's a server based appointment scheduler. Entourage, on the the other hand is made soley for personal email.