I don't profess to be an expert, but I would say that there is next to no benefit to defragging a flash drive. As a solid-state device, it has near-instant seek times: with spinning disks, it takes a lot of time to access data spread out over the disk. With solid-state technology, there's no real overhead in doing that: a file in 1,000 fragments is pretty much as quick as a contiguous file.
(To get real nitpicky, solid-state disks have a finite number of writes that can be performed, so you could argue that you're wearing it out quicker. But the number is generally something large, and I've never heard of a flash drive going bad, to be honest. It'd be like arguing that walking out to the mailbox unnecessarily wore down your shoes: it's accurate, but it's a minuscule difference.)
So I would argue that there's no benefit to defragging flash drives.
(All this said, I have terrible OCD and have done it just so everything looks good. Absolutely no performance gain for me, but it made me feel good.)