There are several camps of design, just as there are several types of user.
There are advantage and disadvantages to either side.
OEMs mix it up among all price points as to whether or not a Card Slot is onboard.
Except Apple. If you look at what Apple charges per GB for extra internal storage you quickly realize they are making a killing just on that one feature.
There are bargain phones without card slots and high end ones with. As a generalization this question does not hold up well.
If you look at your specific example: 535 vs 930 I think it comes down to design aesthetic. Spec point by Spec point 930 and 1520 are equal in everything except screen. Very comparable devices. One with microSD - one without. One with IPS LCD - one with AMOLED. One with a rounded edge, solid polycarbonate body featuring square corners - one with a very straight edge, aluminum ringed body featuring gently rounded corners.
Design philosophy makes the difference.