As an Aircraft A+P Mechanic I can tell you that speed tape is not used to hold things on. It is most often used to cover minor damage. This includes dents, common wind erosion wear or damage to fiberglass/composite panels and fairings (air at speed is similar to a sandblaster), and the covering of missing/damaged aerodynamic seals for fuel efficiency.
It is also used to cover sealant that is still curing. Some sealants take days to fully set and harden, and the speed tape prevents the introduction of water and the force of the air from deforming the sealant. This is often seen around aircraft windshields. The sealant that it covers is for airflow and weather protection, the pressure seal is normally part of the windshield assembly (prefab metal frame that holds the laminar tempered glass or other material). An aircraft must pass a ground pressurization check for leaks before speed tape is applied.
I see comments all over the web where passengers (and even some pilots who should know better) say that "the tape that was holding window/air pressure/wing was about to fall off" and how worried they were.
Speed tape is never used in a situation where the loss of the tape would result in a safety of flight situation. I know many pilots have declared an emergency because of loose tape, and all I have to say on that is pilots are trained to fly airplanes, not fix them.