docbert

IMHO, it depends on the reason for the mayday call.

If it's a mayday call because their fuel levels are getting close to their legal minimum, which means that before too long they potentially won't have sufficient fuel to make it to their alternate airport in the event of a problem (likely Canberra or Newcastle, but possible Melbourne or Brisbane), then I'd say no. In this case they have simply called Mayday to get the attention they need - priority for landing, etc. The odds of an incident during landing are probably exactly the same as any other normal landing, at most ever so slightly increased (due to, say, the pilots being slightly more likely to avoid a go-around if one is needed), so the benefit of the cabin knowing is zero.

If it's a mayday because there's a problem with the plane, then I'd say there IS a benefit of the cabin being aware, and thus slightly more ready to react in case of an emergency. This is the situation where a reminder of the brace position, a double-check of seatbelts, etc, would be in order.

The fact that Sydney Airport apparently decided to treat it as a full-on emergency is just most likely their by-the-book response for such an incident (and not a bad excuse for some real-life training!) - and probably not because anyone truly expected there to be an issue during landing. (Presuming that it was simply a legal minimum fuel issue, and not something more).

Interestingly, over the past few week the route for this flight has been very north on some days. Whilst this could just be due to the weather conditions en-route, it could also be due to them aiming more towards BNE (which is a few hundred miles closer to LAX than SYD) and then once they confirm they have sufficient fuel focusing on SYD. I've seen this style of route before when fog is forecast in SYD - if the fog is bad they are pretty much on route for BNE and land there instead, otherwise they continue on to SYD. It's a much shorter route than getting close to SYD and then having to divert.