The purpose of the holes in the Cross Drilled & slotted rotors are for weight reduction, mainly, & degassing. The slots are for degassing too, but also to help remove the cooked layer of friction material from the pad from a long, hard application of the brakes.
The brake pad is made from fibres and a resin compound to hold the fibres together, (like glue), when the brakes are applied heavily, the "glue" actually boils, and makes a gas, this is what "brake fade" is, feels like you have no brakes, the gas is coming out that much that it holds the pad off the rotor, almost. When slots & holes are added, this gives the gas another out, so more pad is in contact with the rotor.
As for cracking, if the hole could be countersunk, both sides, that is on the inside of the rotor too, then cracking would be very minimal, but this cannot be done, so the idea of countersinking the holes is for releiving a "stress-riser".(same a polishing the beam on conrod, then shotpeening it). Stops it from having a spot to start a crack.
Having air tubes going to the centre of the front rotors will help cooling.
Also, changing brakefluid every 12 mths and the caliper slides working properly, as well as the brake caliper pistons moving the way they should, will stop friction too.