You could apply this logic to anything. A Ferrari isn't worth it if you can get a Chevrolet for a fraction of the cost. (Come to think of it, my wife drives a Chevy.) Both do essentially the same thing. However, if you want a Ferrari, it is going to cost you some serious coin.
Likewise for an SSD, though that might be stretching the analogy. I won't argue the relative worth of buying one, but if you want the performance you are going to have to pay for it. Many here consider it a total waste of money. In my case I was sick and tired of a super slow computer (relatively speaking - I'm comparing it to the computer I built for my wife) and wanted an ultra-fast machine. I talked to a few people that had SSD's in their machines and read a little about them and decided I would pop for one. When I saw a Patriot 240GB SSD on sale, I picked it up. I still haven't put it in a machine yet, but when I do I can give a better idea of its worth to me.
That may be different from the value you assign - or that of anyone else on the forum - but if in the end it puts a smile on my face every time I boot up and I'm willing to part with the cash (which I already did) then it is worth it to me. If there isn't really any performance gain to speak of, then I probably will call it a waste of money. But I wouldn't have known until I tried. I needed a new computer anyways.
That probably doesn't answer your question, but you just have to decide what is important to you and how much you are willing to pay for it. Neither the Velociraptor or the SSD are about value, they are about the ultimate in performance for their respective technologies. Supposedly those Velociraptors in a SATA III striped array really scream - I read it on the internet so it has to be true! - but they are still not an SSD. And by the time you buy two of them, you might as well buy the SSD, save the fact that the HDD's will have more space.
David