My laptop with SSD never gets defragmented. My desktop when I feel like it. If less than half of a hard drive is full, the effects of a defrag are minimal.
One exception: Email, whether you are using Outlook, Outlook Express (RIP), Eudora, Thunderbird or any email client that has copies of your mail. Email files, whether a single Outlook PST or multiple mail files like Thunderbird, get more and more fragmented when you send or receive mail. I set up a special shutdown procedure for a client who had heavy email traffic. Whenever he shut down his system, the Outlook PST file would get defragmented. Before I did this, his Outlook PST file was in thousands of fragments, and you could get a cup of coffee when Outlook started up.
If your hard drive is filling up, say more than 75% capacity, it's time to get a much larger drive, clone the contents, and go forward with the larger drive. Otherwise, just about everything gets fragmented and the system slows to a crawl.
Defraggler is an excellent tool to look at fragmentation of individual files, and you can use it to take action to defrag just a few important file.
Windows' CONTIG command defragments a single file... Ben Myers