I have found that points transfers from Amex for example can be turned into more $$ value with Aeroplan vs. Bonvoy, especially if you are using those points to redeem for long haul PY or J.

Amex transfers from to AC are 1:1 and to Bonvoy they are 5:6...so transferring 100k Membership Rewards points gets you 120k points with Marriott which is worth probably 4 nights at a property in NA that will cost you around $200/night, so $800 in relative value. Meanwhile, you could possibly redeem your 100k Aeroplan points for a "J" ticket TATL that could cost $4k and then stretch those points even further if you have a Priority Reward to burn so for me the math is pretty easy.

I've seen sometimes there is better Bonvoy redemption value at some European properties or as boomerfss suggests when you get those occasions where the cash rate is absurd for whatever reason but the property can be had for a modest number of points.

Basically, I choose to earn Bonvoy points from staying at Marriott properties but don't go out of my way to earn them in other ways or transfer into the program when I can get such better relative value with Aeroplan.