Keith Hill's in-form side had been harbouring thoughts of a second successive promotion but striker Simpson brought them back down to earth with 11 minutes remaining.
Sam Saunders raced down the right-hand side and flighted in a teasing cross which Simpson met with a firm six-yard header.
Dale had earlier fashioned a series of chances which all went begging.
Will Atkinson fired wide from 12 yards and then skipper Gary Jones went close with a rasping 25-yard strike.
However, Brentford held firm and snatched victory through Simpson's predatory late strike.