While I agree with what has been said, I think an important part of it is price differentiation.
Air Canada has been amazingly successful keeping a wide degree of price differentiation. Right now, a few weeks out, AC will sell an economy ticket YVR-YYZ for $310 in Tango, about $500 in Flex, and $1688 in Latitude (Latitude:Tango ratio about 5.5X). Business lowest starts at $1250 or so, and flexible business about $2650. In comparison, UA prices cheap economy EWR-LAX at C$377 and flexible economy at C$1201, a ratio of about 3.2X, significantly less. [I pulled these off of the ac.com and united.com websites right now]
Keeping people willing to pay 5X+ for the same seat requires first of all market power, which AC has in Canada, of course. But it also requires maintaining at least apparent product differentiation of some sort. You've got to make Flex look sufficiently inflexible for people/businesses to be willing to pay for Latitude.
Of course, pure yield management/fare bucket availability gets you part of the way there. Change/cancellation fees "help" too. But any additional differentiator, like a $75 SDC helps. And the value at stake for AC is not just $75, but the Latitude-Flex differential.
Flyertalkers are of course quite savvy and see the smoke and mirrors: it takes a lot of change fees of any kind to make a high fare "worth it" and we generally don't buy Latitude with our own money. But as an independent consultant, I continue to be amazed at how many of my clients have no trouble at all paying whatever ticket invoice I present to them, any fare type, but balk at the clause I put in my contract terms that says "nonrefundable and/or restricted tickets can be purchased if Client agrees to reimburse change, refund, reissurance, or other service fees as needed, and/or nonrefundable portions not recoverable if travel plans change".
So when I fork over $75 of my own money to get home 3 hours early from a rained-out weekend getaway, I realize that's collateral damage from AC's successful campaign to get my clients to pay $$$ instead of $$ for my plane tickets, and therefore make it a lot easier for me to get to 20,000 AQD than it would otherwise be...