As for Arashin Cleric specifically, the reasons I advocate for it are:
A) Easy to cast
B) 2 mana, so can come down T2 without relying on a dork
C) 3 toughness, which means it can block Goblin Guide and a single prowess trigger on Monastery Swiftspear. Also means it is very burn resistant with any lord effect/+1 counter.
D) Consistent value at any point in the game
E) Aside from painful mana draws, landing the instant 3 lifegain can often put the game out of reach.