A conventional defrag will not help the random read and sequential read/write performance of an SSD much. However, free space fragmentation can reduce the random write performance of SSDs* So free space consolidation can help to minimize the chance of random writes at both the logical disk level as well as the SSD's physical cell/block level.
It's important to remember that the 'internal fragmentation' i.e. wear levelling of an SSD is a function of the drive's firmware and has nothing to do with the filesystem fragmentation which occurs at the logical disk level. The SSD's hardware is abstracted from the filesystem i.e. the file system does not know whether the drive is a SSD or a HDD**.
*Which is their weak spot anyway because of the mismatch between the logical cluster size and 'physical block' size (4 kB vs 512kB) requiring frequent erase-rewrites for entire physical blocks.
**Maybe different for Win 7 since it is said to have the ability to detect/recognize SSDs and disable defrag accordingly.