Trolls could also have been used for the heavy work in Morgoth's dungeons and under-ground mines/caverns in Utumno. That way, they would never see the light, and Morgoth had no way of telling what it would do to them.
Also, after the first Sun rise, and after Fingolfin marched into Beleriand, it is said that the Orcs were dismayed (and a lot of other stuff). Wouldn't Orcs be dismayed if they suddenly saw the big guys (i.e. Trolls) suddenly turn into stone?
The Sun could have also brought out their true nature. Morgoth made them in mockery of the Ents, and out of stone, so stone is in their nature. Light has always been an element that brings out the true nature in things. That is why all evil things (or at least most of them) are characterized by a love of the dark, because that darkness hides their true nature from both their friends (if they have any) and their enemies. Thus, once they go out into the light, their true nature, stone, is revealed, and they turn into stone.
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But Melkor also was there, and he came to the house of Fëanor, and there he slew Finwë King of the Noldor before his doors, and spilled the first blood in the Blessed Realm; for Finwë alone had not fled from the horror of the Dark.
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