If you think you can use less than 200MB per month, it's worth it. If you are going from unlimited to 2GB/month, it's worth it but only if you never use more than 2GB per month.
If you are never going to use >2GB, then I don't think it's worth paying $5/month to keep the option of havingit. Particularly not since, in the unlikely event that you go over, it's only $5 more per GB that you go over. So if you save the $5 per month every month and if you do somehow manage to use 4GB, it's only $35 that month...
I looked at my mother's iPhone and she never used more than 65MB in one month going back for years and her average was like 30MB. So I switched her to the 200MB plan and she's never received an SMS warning her that she is close to her limit. This saved her $180 per year... and cost her nothing because she's never been remotely close to the 200MB limit.
For myself, I tend to use about 250MB... which made it more questionable because the savings from unlimited to 2GB were not much. Still, I switched down from the unlimited to the 200MB plan and I track it more closely and I occassionally go over on trips. In which case I bump it up to 2GB for that month and then drop it back the month after. I saved closer to $100/year.
Still, I know the feeling... it's a one way trip and you think "what if I need this data some day"... only you can answer this question... To me, I never regretted changing plans... but I still remember that feeling when the AT&T rep said "ok, when I make this change, you can not go back, are you sure?" and I paused a bit before I said "yes".
There are a lot of people on here who stream music and videos and stuff and for them, I would never recommend changing. Stay with unlimited until they pry you off the plan kicking and screaming... it was like the Sprint SERO people... it's a good deal and stick with it if you can. But if you are only using like 500MB per month, what's the point of paying $5 more in the unlikely case tha you will quadruple your usage one month... particularly since it's still only $5/GB after that.
It might just be $5, but $5 here and $5 there and soon enough you are adding up to real money.