In AD&D (or, presumably, 3E) where the cleric has a lot more spell slots (both generally, and due to high Wisdom) and a wider variety of spells to choose from, allowing them to cast spontaneously would be too big of an advantage (IMO), but in OD&D where the cleric is, as you note, a distinctly second-class spellcaster (remember also that they can't use the wands and staves that give mages a lot of their firepower (and are, in fact, their main source of firepower -- a mage isn't able blast lightning bolts every round because he has a ton of 3rd level spell slots, he's able to do it because he was a wand of lightning bolts or a staff of power)) having free choice between 4-6 spells for their 1 or 2 slots/level doesn't seem like a game-breaker. I can also see only allowing spontaneous casting for level 1-4 spells and demanding that 5th level spells be prepared in advance.