Many use altimeter based dual deployment because of better accuracy and control of deployment . Specifically,you can set drogue deployment at apogee or a few seconds later. Also, if you use 2 BP drogue charges, one can be set to fire before the other to more ensure deployment via this redundancy. In addition, depending upon conditions (e.g. high winds or not) you can set the main deployment at a lower altitude to prevent too much wind based drift.
When relying on motor ejection only (single deployment), unless you have experience with drilling the ejection charge, the ejection charge is approximate and if too early can cause shredding of parachutes and if too late, a hard possibly damaging landing. Some commercial motors have long delays that even with drilling are too delayed to sustain a safe recovery.
Some also do altimeter based deployment and also utilize the motor ejection as a back up to the altimeter based drogue deployment.
I hope this helps.
Fred, L2
ICBM,S.C.
KG4YGP