I'm on a grandfathered unlimited as well. The big issue with my parents is signing them up for a new account they would get limited. So then they have to watch their usage to stay under the limit or go over the limit and pay the extra $.
That itsself is a hassle, espically when web sites think they need flash and videos and are designed for uncapped networks. Its hard enough to stay under a limit on a phone let alone on a computer. My wife has went over her 4G limit a couple times.