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It’s not a good enough deal to transfer points unless you were already planning to transfer points there.
The flexibility lost by transferring to one particular airline vs. leaving the points with Amex until you have a particular redemption you want to use them for is not worth the 10% extra miles.
Even the 25% or so bonuses that they have to Virgin or Flying Blue sometimes often aren’t normally worth preemptively transferring points.
Of course, the bonuses are great when you actually do have a need to transfer the points to that airline, though!