
partyrico |
A quickened 0th level slot would take up a 4th level slot, which a cleric doesn't get until he/she is a level 7 cleric. A quickened 1st level slot takes up a 5th level slot similarly. This makes quickened spells not worthwhile until the higher levels, when characters have enough spell slots to spend higher ones on more actions.

partyrico |
I'm sure there are some corner cases, ghost sound with threatening illusion and quicken lets you threaten but you can move it around with swift actions? A quickened create water to put out multiple characters on fire so that you still get your standard action? Swift action prestidigitation and use standard action to ready and action to cast it again when they remove it? There are some weird uses but most probably aren't worth the higher slot