I would leave the leaves in place to act as mulch and decompose to add nutrients to the bed, unless you are talking about a thick layer. With a thin layer of leaves, the only possible downfalls would be aesthetics (if you didn't like the looks of the leaves) or disease (sometimes mulch can provide a place for fungal disease to reside), but, unless you are aware of a disease issue in your beds, the benefits should outweigh the negatives.

If you added lots of leaves, they could form a barrier to proper drainage and aeration, and provide an even higher potential for disease retention. If lots of leaves are present, you might consider removing some of them. You don't want a thick, solid, impermeable mat of leaves.

Finally, if you have woody plants in your beds, make sure the leaves are raked away from the base of those plants so as not to touch the bark. Leaves, or other types of mulch, should not be placed up against bark.

I agree with Brandon but want to add one more thing, if it is Oak leaves they can take a long time to break down. I would remove the oak leaves and shred them, then either put them back or compost them.