SIA, good points, some I hadn't thought of.
It depends on where you see your career ending; in most cases for early-to-mid-30s level 9 Flt Lts like myself, the extra 2 years or so will give you a noticeable uptake in pension by enabling the AFPS15 EDP.
On the other hand, if you don't get a PA offer this year, it's certainly a negative as you've delayed entry to the pay spine by another couple of years.
Pension calculator seems little use, as I don't believe it really understands "future PAS" that well yet. Marking time for 6 years at level 9 waiting for PAS is bad enough, adding 2 or more years to that seems pretty untenable.
It only seems to make sense, with hindsight, for people who were definitely sure of leaving at their option point, and who'd like the option to stay around for an extra couple of years to elevate the pension slightly. If you have PAS ambitions beyond this year's board then it's a bit of a negative...... promotion I guess it wouldn't affect, but that's its own conversation!