The body needs delta sleep, science seems to be pretty sure of that.
The arguments about the brains need for REM sleep in adult humans is ongoing.
They believe that REM sleep is necessary for brain development and that the brain continues to develop as late as 20.
Dolphins are born fully developed apparently and they have no REM sleep. Human infants have a much larger portion of REM sleep compared to adults. In mammals, the less developed the brain is at birth, the greater the percentage of REM sleep that species has.
The consensus seems to be that REM sleep is necessary for brain development and the ordering or memories and they believe that dreams are a result of that memory ordering. But then I have read a study that indicated we dream during all stages of sleep
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