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The argument for waiting is that there is a high possibility a fellowship trained anesthesiologist would never be required to take the test. The trade off, as you point out, is that the test may be harder if taken in the future.Additionally, though I didn't explicitly state it, I took it this time because I believed a larger number of 'older' anesthesiologist were going to take it (and there being a generous curve,) Why wait until there is a less generous curve and you're competing with a greater proportion of test-takiers that are fresh out of fellowship.
For those seeking cardiac certification who have not completed a fellowship, I can understand the motivation to take the test ASAP due to the grandfathering timeline set out by the ABA.