Yes nami can POKE but she can not ENGAGE a fight like what you were implying in my mind. Janna poke is minimal, and often has her ending up on the short end of the stick if they have any engage champion like thresh or blitzcrank. Also enemy adc tends to have warlords most often and are more than willing to poke as the janna tries to poke. The shield takes a bit but you run the risk of both the enemy adc and support hard engaging you........ While your adc is just farming.
Nami has easier poking for sure, a little less issues, but again, if the enemy has a hard cc hard engage champion, then if they are able to land their shots..... The nami is going to be hurting and needing to stay back a bit.
How they work best, like i said, is when the adc engages. That forces the enemy to either try to engage or back off. You just said the exact reasons why i said those adc champions work well with those supports. So im not sure if you are trying to argue against me or help validate my comments.
And yes i am a mastery 7 nami, i do know how to play her, i do know how to poke and dodge with her, i do know that when it comes to actual fighting and engaging, it helps a lot when your CC isnt used just so the adc can get to the enemy..... Unless the enemy is playing so you have to. It is best used in the fight when your adc is able to lay down damage DURING the bubble stun and ulti knock.