Cut-off frequencies for common mode and differential mode filter?
why don't we consider C3 and C4 for the corner(Cut-off) frequency
  calculation


It's a simplification. Consider the following; all caps the same value and both resistors the same value. The differentially placed capacitor C2 and R2 and R3 form a filter that might have a cut-off of (say) 1 MHz.

As individual wires, the cut-off frequency has to be 2 MHz because only one resistor (rather than two in series) affect the cut-off point with their respective capacitor (C3 or C4).

That's the first simplification.

Next we have to consider which is more important to protect against - common-mode or differential noise. A perfect diff amp (without filters) is theoretically capable of dealing with common mode noise so the filters formed by C3 and C4 will be redundant. However, the perfect diff amp will still amplify differential noise so, more emphasis is placed on C2. In effect, C2 will usually be significantly greater in value than C3 or C4.

This means that it is a fairly reasonable assumption to make that differential filtering is down to C2 whereas common-mode filtering is only performed by C3 and C4.