Whilst they carry fuel, I can't understand why they would be required on the short hop between Belfast and Isle of Man as surely they would add weight to the aircraft thus requiring extra fuel. If the aircraft can operate without the wingtip tanks as shown by Sundays photo then it really should be operating without the tip tanks on a daily basis 
The Tip tanks are standard fit on the LET 410 and you are quite right the aircraft can operate happily without them,however it`s all a question of operating costs.
The Manx2 aircraft usually refuelled at Aldergrove after the morning arrival-the aircraft also operates flights between IOM and Blackpool,perhaps you weren`t aware of that.
Basically the less times they have to refuel the better,as it keeps operating costs to a minimum.Manx2 have very tight turn around times (as most budget airlines do)15-20Min's was the norm.
The crew told me that sometimes the tip tanks caused handling problems when taxiing in strong winds so maybe they were removed for the winter season when high winds are more likely-that would be my guess.
