I use fifth gear on a motorway / dual carriageway if I'm in 6th gear and I need to drop a gear to accelerate to get a space to either get off at a junction, get in front of a lorry as a crawler lane comes to an end and I realise the road is running out. Although 6th gear is OK, I get more urgency out of 5th, and 4th gives you too little space before you hit the red line and you're changing again.