As someone else pointed out earlier, if you're purely a domestic traveller, alliance benefits don't count for much. (Particularly since you can get MLL access and priority check-in for CAD $175.)

But if your travel is split between domestic and international, you're still limited on your domestic benefits, though you might be able to use some benefits of the alliance carrier while travelling internationally.

Even if your travel was stictly international, there are all sorts of complications and extra inconveniences. If you wanted to join UA's program, you'd most likely have to connect through the US in each direction, adding immigration fees and the usual border crossing hassles, and there are fewer departing flights to any given destination. It's also harder to earn partner miles when your home program is in another country (note the allowances AP had to make for UK members and expiriing points.)

Because of those extra impositions and limitations, for many people who want something more than and economy seat to their destination, there is no real competition.

Which I suppose is a way to say that perhaps while AC might have some competiton, Aeroplan has much, much less, and therefore can easily chop away benefits as if they had no competition.