The disk still gets fragmented, it's just less likely to be. And while it works great for files that are created on a specific files, it doesn't mean it'll work that great for files that grow dynamically.
It also slows down file creation if you've got a lot of small files. Fragmentation is also likely to occur in systems with large amounts of small files. Email servers where mails are stored as separate files are a good example.
Linux does a bit of pre-processing while you're actively working. Windows allow you to defrag at a later time when the system is not as stressed for processing time.
I believe there is an online defragment util in the works for ext4.
i never defrag... y? cos i have a program that constantly defrags my pc as it CPU usage stay below 30%. the program is called IObit SmartDefrag. its free.