The method MissHostie describes is pretty much it. If crew can get them to a floor-level exit, it's down the slide they go (with the CC member sitting behind them, legs over theirs, and arms holding theirs crossed over the chest) to secure them down the slide... preferably with others to catch/assist at the bottom. If no slide/undeployed, then it's pass them out the best way you can, hopefully you have ABPs, if not, harsh but true a broken ankle is better than dead, I guess you just have to drop them out as gently/best you can and get them moved away... I'm sure in any evac situation you'd have pax already outside you can scream at to come and take the person if need be...
It is sad but true that each case would be on its own merits, if the crew felt they would imperil their own lives by going back for a disabled pax, then it's their decision not to do so (one which hopefully I or any other CC would never have to live with)
However, if they felt it was safe then you have to weigh up the risks... (a potential to cause further injury vs a certain horrible death) would clinch it for me... if they were going to die and they could be saved with little risk to myself then I guess the decision would be to save them... can worry about causing further injury later... as one instructor said to me, better off injured than dead!! At least they have a chance... and no, we are not qualified to decide if someone should be moved on medical merits... it's more a 'Will they die if they are left? Can I evacuate them without harming/killing myself?' If you can do it, go for it... otherwise if you know help is on the way it may be better to leave it to the professionals... any way you put it, it's a tough call... but I'd hazard a guess not much time is actually spent thinking about it when it actually happens, but decision is made in the heat of the moment without 'conscious' thought... if that makes sense!!!
All the above goes to show the importance of a thorough, detailed briefing and ensuring the pax understand what you are telling them!!! I reccomend anyone who cannot assist themselves off the aircraft to have a comapnion who can assist them if need be.. at the moment it's not madatory for all wchr pax but maybe it should be, to give that extra margin of safety... after all, who better to help them than someone that does it all the time...
As for stretcher pax, I think these days (in Aus at least) most of those are taken care of by RFDS/ air ambulance rather than commercial carriers for the above reasons... e.g they have a nurse to look after them should there be an emergency
Ok, that's my 2c....