I'm not going to go off the deep end and say this is a must have series, but there is no doubt that this series with the Cardinals can help set the tone for the Reds this season.
The Reds need to send a message to the Cardinals that it's their division, and St. Louis will have to go through them if they want it. Sure Pujols, Larussa, and Duncan are gone, but most of the pieces are the same.
The Reds need this for their own confidence as well. While they have matured and some of the young guys are coming along, they've not had any sort of consistent success against the Cardinals. They need to know that they belong and they can get it done against them.