The fire brigade or other public safety people give out instructions like this because they are looking at preserving life in the aggregate. Panic striken families clogging the single stairway makes it more difficult for first responders to respond.

I've been involved in a few minor disasters in 15-25 story high rises. One was a roof fire, another was a kitchen fire halfway up, another was a catastrophic leak of water and glycol from the heating system, and the last was a thick cloud of smoke from a nearby oil-fired power plant that suddenly shutdown during the blackout in 2003. Guess what? There were catastrophic fuckups in every event (fire alarm failure, water pressure failure, firefighters sent to the wrong place, etc). But the SOP was always the same regardless of the situation -- stay in place.

At the risk of admitting to being a selfish asshole, I'll come out and say -- fuck that. I will be bugging out, and I'll tell anyone I know to do the same. I'd rather die trying to get out than getting trapped like the poor souls in this building or WTC.