No, they're not a gimmick.

PG injections and a cidr work in two different ways. As t-boss mentioned, only a percentage of females will respond to a single PG injection because of where they are at in their cycle. Most would not consider the results from a single PG injection timed AI protocol a success considering the cost of the wasted semen, labor, and lower conception rates.

If I have facilities and time, or when semen costs are a concern, I use the initial PG injection to save me a little money on the cows who happen to be already primed and then follow it with the 6 day cidr.