Based on experience (being SE for the last 2 years), I would say the benefits are as follows:
1) Phone gets answered on the first ring when you call to make reservations.
2) Phone gets answered on the first ring whe you call to claim Aeroplan awards.
3) You are able to get exactly the flights/dates/times you want when you are claiming Aeroplan rewards (there is probably some fine print somewhere, but I have always been given exactly what I want).
4) You may use the MLL whenever you wish, regardless of what fare you are travelling on (heck, you can even use it if you are not travelling).
5) The airline gives you lots of upgrade certificates, and lots of additional bonus aeroplan points.
To me, these are the main advantages.