Three possibilities for this decision present themselves IMHO :

1. Rudd genuinely believes the economic component needs to be 
addressed during question time in the house of reps
2. Rudd genuinely believes PG is hopeless at answering questions on 
curly issues full stop
3. Rudd genuinely believes PG is not good at towing the party line 
and will answer questions honestly according to his own understanding 
of the matter at hand.