Why are there empty spots on the 40-man roster?
There are currently 37 players on the 40-man roster. Brendan Donovan is likely to be moved to the 60-day IL, which would open up a fourth spot. Why aren't the Cardinals taking advantage of this roster space? Plenty of exciting players who could help the big league club shortly, such as Michael McGreevy and Masyn Winn, are currently not on that roster. They can't join the team until they're placed on the 40-man.
Unfortunately, these empty spots are only temporarily empty. Ryan Helsley, Wilking Rodriguez, and Packy Naughton are likely to rejoin the roster over the offseason, if not sooner. The Cardinals do not want to lose any of these pitchers, but adding players to the roster now would likely cost them their spots.
The roster crunch that would occur should the Cardinals add players could force them to try their luck sending someone through waivers. This likely isn't a risk the Cardinals will take, meaning they won't be using these extra spots anytime soon.