Koshinage is like the spinning back kick in Karate.
It has a big wow-factor but also a much greater risk than other techniques of going horribly wrong.
For Judo players working in close in competition it makes some sense.
For Aikidoka in a self-defense situation (martially-effective), it is just plain stupidity. Any place the energy would be right for one, a timing throw would be quicker and safer.
It's only going to be the best solution with either a stupid attacker or when there is a blue moon over Aruba.