The Mets have the luxury of a large lead in September. Combined with the Nationals playing poorly, the Mets can afford to experiment with the roster.
As of today, the Mets have managed to build a 7.5-game lead on the second-place Nationals. This lead has been forged by an improved Mets team in the wake of the trade deadline, helped in no small part by a Nationals team that hasn't seriously threatened the division lead for some time.
As a result, the Mets have the luxury of tinkering with their roster as the season wanes.