The new timing engine in Logic Express/Logic Pro 8 and later allows automation nodes to be placed much closer together than in past versions. In some cases, placing clusters of nodes too closely together will place a heavy load on the CPU, resulting in audio drop outs or stuttering. There is usually no audible advantage to placing automation nodes closer together than 1/96 note.
When you write automation by moving a fader in Logic's mixer, or from a control surface, nodes will automatically be spaced far enough apart to avoid stuttering. When you use the pencil tool to create nodes, they are limited to a spacing of at least 1/96 note. It is only possible to space nodes more closely together by clicking in the automation lane with the arrow tool at very high horizontal zoom settings.