If we take Lucian at level 6, with no mana items and no manaflow band, he has about 500 mana
If we give him 1 point in Q, 1 in W, 3 in E and 1 in R, ... (this is the lowest possible mana consuming skill order at this level, meaning any other order will be worse for him in terms of mana consumption) ..
Q costs 50
W now costs 70
E costs 20
R costs 100
If we want to ensure that he is able to use his ultimate, we immediately subtract 100 mana, leaving him with 400 mana to use any combination of his other abilities for a rotation.
W is now a much more significant burden on his rotation pool.
This means he has to either be more conservative with his safest poke ability, or he has to invest gold (or a rune) into obtaining more mana / mana sustain.