why do people use heli coils in cylinder studs.  they never hold for long. 

I have used time-serts and heli-coils.  I prefer the hel-coils in aluminum.  The majority of the failed heli-ciol repairs we find are because shops use heli-coils that come with the repair kits.  The helicoils that come in the kits are typically 1.0 or 1.5 D heli-coils which are too short.  A 1.5 D heli-coil only has a length of 1 1/2 diameters of the bolt that screws into the new insert  That is ok for repair of a hole in steel as long as it is not something that has extra high loads like head bolts, main bearing caps etc. 

We use heli-coils that engage the full length of the head stud.  The length of the head studs threads are usually about 2.5 times the diameter of the thread.

We repair a lot of time-serts that are loose in the hole and back out even though they have been "staked" or flared.  We also repair stripped time-serts.  None of the time-serts or heil-coil repairs are idiot proof.  Novice mechanics think it is better to over torque a bolt to just make sure it does not get loose.

Most of the time when the threads pull in the original casting, it is because someone over torqued the head nuts.  Mechanics over-torque the head nuts because they do not use a torque wrench or they think that putting a little extra torque will fix a leaking head gasket.  If you have a leaking head gasket replace the gasket and machine the gasket surface if it has blemishes or is not flat.