The thought of special engines for qualifying is long gone, I hope, but allowing only one free engine change in the whole season is a bit pointless if a manufacturer's engine has an inherent weakness.
Their engine can't be improved during the season as development has been stopped so potentially the team are sitting ducks. If they are allowed to change the power plant regularly with less or no penalty they may be able to run with different settings in the race and complete the course. If too many teams are hampered by reliability, can't work on a solution and lose points, we may see one or two throw in the towel.
Aagrh!...... I'm forgetting of coures a limitless supply of engines may cause a problem with the budget cap that is due to come in.