I don't mid main either, however, manaless champions have more poking ability while having low range and high CD.
Champions like Katarina do a ton of damage, but has longer cooldowns than say Lissandra when it comes to Q poking.
Manaless champions also require the need for spell weaving for maximum damage, where as Mana champions have all their damage in Q or R. Think of Akali where she has to Q and AA to deal damage, wheras Lux can throw a Q, and deal a lot of damage.
As far as manaless champions go, the only ranged one is Kennen, and he has very short range. Yasuo, Zed, Katarina, Akali, etc, are all melee; Mana champions like Ryze and Azir have ranged auto attacks.
In the meta since S4, assassin champions have gained an ability to quickly execute low enemy champions. Therefore, if manaless champions can poke you down for a long period of time they can eventually take you out. Mana champions are dependent on mana and eventually become powerless to stop energy/manaless champions after running out of mana and going low on health.
Basically, if the enemy laner can outsustain you, they can eventually outpoke you and execute you. This can be countered by Mana dependent by waiting till maybe level 3-5 to hard poke enemy champions. When the poke ability reaches it's second rank, that's the best time to use it, and not before.
Ahri has pretty high mana costs. Poking with Ahri is only efficient with her Q, and not spamming any other ability. By saving mana for Q skill shots only, and selectively choosing when to strike, this can maximize Ahri's use of her Mana. This could go for all champions.
That's about all the information I can spew at the moment. I hope this helps.