I think the people who are saying our SP is already stable and hasn't been an issue so we don't' need a SP are probably worried about winning the division and not making any noise in the post season.
Need a legitimate #1 or #2 starter if you have any chance at winning playoff games. A playoff rotation of say Castillo, Gray, Ryan at the front of the rotation is the minimum you'll need. Also strengthens bullpen by allowing Ober, smeltzer, ect.. to throw out of the bullpen come postseason.
Winder, Ober are fun young guys with upside who hopefully are longterm answers for us but when you get to the postseason again Houston, NYY or TOR they have proven pitchers with really good lineups. Need an Ace to go against those lineups come postseason.
Winder, Ober, Smelzter round out the rotation nicely to win a bad division and be ok with no post season success.