Aye, that's where I'm getting screwed. I guess it depends how you classify "saving money" because at least in my data-light usage case it doesn't make sense to pay more for data I won't use, to pay less for the phone. I guess I'm not a class of customer that really makes buying worth it. Still, if it's marginally cheaper I guess it's still something to go for (especially as I dunno what the replacement options are if you break the phone, but as my 4S is still in perfect condition it seems moot for me.)
seriously, look into it. My brother thought THE EXACT same way you do... EXACTLY. He looked into it and found that he'd save $19/month (ish) going to 10GB. And he and his wife use a whopping .9GB. Yeah, 900MB. So what? You have a shit load of data you will never use. In his case that's still $228/year he's saving.