There is 1 defender per scenario ... to see 1 in 20 runs you doing something wrong.
Every run I have killed 1 (bonus quest).
Not all of them drop kits... but every scenario have 1.
The named (human) elite defenders always drop a kit, the droids do not.
In the 14 solo-scenarios I farmed tonight, I saw a named defender 3 times and got a kit from all 3. So I'd guess the drop rate is somewhere around 20%, as it is with similar currency drops.
It would be interesting to know if groups have a higher chance of getting a named defender. If not, since you'd have a 1-in-4 chance of getting each one, your actual acquisition rate could be as low as 5%. But since they are relatively easy to farm for geared 78s, I'd guess they will be readily available on the GTN for 30-40k each in short order.