I think you will find there is very little rigidity in UPVC frames, even if they are reinforced. I believe it is customary practice to 'brace' the glass against the frame using packers in the way your fitter has done to ensure everything sits properly. This should have been done right with the packers in the correct places in the first place. Similarly the wall fixings have to be in the correct place with the correct packers used to ensure the frame sits square and plumb in the aperture (with a 5mm min gap all around as well of course). Inherently UPVC frames are wobbly things and need the glass to be corectly installed/braced to provide rigidity.