The voltage requirements at the MMC card inputs (MOSI, CLK, /SS) are defined in the MMC card product manual:
VDD in this case must be between 2.7 and 3.6v.
Input HIGH voltage:
min: 0.625 * VDD
max: VDD + 0.3
Input LOW voltage:
min: VSS - 0.3
max: 0.25 * VDD
As long as the voltage tapped off the divider falls within this range, all should be well.
The document to read is here:
http://www.sandisk.com/Assets/File/OEM/Manuals/ProdManRS-MMCv1.3.pdf