Do SSDs need to be defragmented?
No, never. Defragmentation is helpful on spinning platter-based drives, if the filesystem doesn't handle it (which HFS+ generally does, on a Mac), because it takes time for the disk to spin around to the right spot, which has to happen more often if one chunk of data is split between several locations. See Access Time.

A solid state drive has no moving parts, so there's no mechanical delay. Reading a section of data takes approximately the same amount of time no matter where it's located in the flash.

Defragmenting a solid state drive is actually harmful because it forces the drive to move data around for no good reason, and flash chips have a limit on how many times they can rewrite a section.