What did they want Rakan's identity to be?
You have this champion with a kit that sort of screams at you "Go in, go hard!" and yet he's got incredibly squishy base stats, he can't take relic shield because of his (craptastic) auto range (THRESH can make relic shield work and he's got the same range as Lucian!), and as a tank his heal and shield are lackluster.
Let's look at the other side of the coin. He's got a great AP scaling heal and shield, but the heal is still relatively short range (I can't tell you how often I throw a Q out and the particle clips the enemy but instead of landing it just disappears) , his shield got nerfed because they "don't want him to be a better enchanter" than Soraka/Janna/whathaveyou, and he's overall not good in either role.
He's delegated to taking ancient coin first, being squishy half the game, then late game being a somewhat mediocre bruiser in a support role.
work due to his e passive which gives him extra e damage to last hit minions
he can heal his adc(even if its 100-200)
he can tank damage for a while and jump in and out of a fight
he can cc an ENTIRE TEAM point and click