It takes up 5 times more memory and has more security vulnerabilities instead of less.
Nearly all the people I know who have the annoying MSN virus(that makes people send random messages generated by spyware) are using the latest version and antivirus software at all times.
Use the mess patch (https://mess.be) on MSN 8.5 to remove anything you want from the UI, you can also use the regular settings window to remove the rest. And of course Pidgin is an awesome alternative which I use on my slower computers.