This is a very interesting topic .
Thanks to potential/williamd for their first hand accounts .
A couple of points from a flight crew perspective .
As has been stated the decision to evacuate cannot be taken lightly the cabin crew have for sensible reasons strict criteria as to when they can individually initiate an evacuation themselves.
Passengers unilaterally initiating an evacuation themselves could have disastrous consequences ( engines running / exhausts / wheel fire are just a few ).
Communication between flight and cabin crew is vital the captain cannot possibly know the exact extent of the threat immediately in a rapidly deteriorating situation .
When distractions are present hearing is one the first senses to be lost also PA speakers are mounted below the overhead storage ( mostly ) so people standing in the cabin are less likely to hear PAs especially if there is noise in the cabin. Williamds' evidence would support a PA was made and that the flight crew were conducting a logical decision making process whlst balancing the risk of injury during an unnecessary evacuation . Its a judgement call and it seems to me to have gone just about right.
Ironically the present refresher for 747 BA flight crew involves uncontrolled smoke in the cabin leading to an evacuation but from the downwind position in low visibilty procs. rather than during taxi out . I'll put it into my lesson plan from now on .
We can all be experts after the event and this incident will no doubt hone SOPs in a time critical environment .
It's great everyone got away with only minor injuries / inconvenience ( although not inconsiderable )