As PIC, you don't know what is going on back there, once you've made the ultimate decision to blow the slides (or jump in the case of the SAAB), that's the end of your authority. You don't get to say "Oh, we genuinely believe we're on fire and need an emergency evacuation, but only use 1 of the available exits", particularly having regard to certification requirements that say everyone should be off within 90 seconds once the command is given.
Actually the PIC has radio comms with ATC, possibly ground staff and fire marshals at the primary airports. So the PIC will have inputs from all angles as to whats going on outside if done properly. And yes the QF brake fire combined with whatever else could have been going on could have become much worse very quickly under the right circumstance, after all the fire is located under the engines and wings, would not take much for it to spread very quickly with an ignition source that takes possibly hours to cool down. I noticed the QF aircraft did have the R1 exit open, but all passengers were exiting through L1 with bags, so a confusing picture to say the least.
If the QF situation had occurred at an outport without nearby fire service there may have been a lot more damage. There was a significant amount of flame coming from the tyres before the FO sprayed it, the BCF only had a few squirts left with the right wheels smoldering as well.
If the situation is confusing I say get all the info you can before moving, however if unsure I don't think the SAAB did anything wrong once they wanted to empty the aircraft as the FA could quite easily have ordered other exits used as well should the situation deteriorate. Very different to the QF 330 on the stand where pax were actually exposed to hazardous fumes.
Evacuations are fluid and you should be able to adjust to the situation with what little control you have.
You can play "what if's" till the cows come home, but, to use your example of the QF brake fire, a brake fire, while it can be visually quite spectacular (same as this incident, I guess) does not typically require an emergency evacuation. Nonetheless, I agree in principle with the point you make, however giving such a direction doesn't permit you to place your crew and passengers at risk, which you do by default if you order the prohibition on using otherwise safe & serviceable exits.
Evacuations are exactly playing with "what ifs" as you are expecting things might get to the stage somebody is hurt, after all you evacuated for a safety based "what if". Almost every evacuation is going to be slightly different so there's no procedure that covers all outcomes. We have standard Evac procedures for the worst case scenario where you need to get everybody off ASAP, but that may not be the case in 90% of Evacs. The Bangkok golf adventure proved the standard procedures were flawed to rely on the Captain at all for Evac notice and procedures changed and so they will keep changing as we find new holes in the cheese.
One thing with both the QF incidents I mention is that both crews were not following the procedure for the relevant operation, if they did an Evac would not have been warranted. The 330 should have not been using the APU and the Dash 8 is yet to come out but sounds like it should not have taxied to the terminal under power. We all make mistakes, this situation just sounds like another one.
At what point would you evacuate? When it breaks in through the window or use forrward exits while it's chewing the tail?