Short answer: only if an overnight genuinely isn't possible.
A day trip from Adelaide covers Seal Bay and the eastern part of the island, but Flinders Chase and the Remarkable Rocks on the western end are a long drive away — most single-day tours have to choose one region over the other.
One night lets you see both ends of the island without rushing, and gives a real chance at spotting wildlife at dawn or dusk when kangaroos and wallabies are most active. Two nights is ideal if food and wine tastings are part of the appeal, since the island's producers are spread out and worth lingering over.