When data is written to an SSD it is scattered across the memory chips, in a sense it is very fragmented but this doesn't affect it's performance, unlike a traditional HDD. Defragging an SSD can reduce it's lifespan, as they have limited write cycles.
@Soulres: Installing an SSD is not about improving frame rates, and I never said it was, it improves load speeds and disk access times; this can reduce pausing and stuttering caused by disk access in-game. As I mentioned, extra RAM also helps.
Sure, you don't need an SSD but when you see your PC boot up in under 20 seconds instead of over a minute, you'll never look back!