I've never had a problem with shared boxes personally, they offer decent speeds at an economic price point for many users. That being said, I have sometimes felt concerns over the shared I.P. address commonly used. For example, what happens when another user on the box with the same I.P. is using it on the same tracker you use, or perhaps even the same torrent? I've personally never had an issue from this on the trackers I currently have.
But dedicated boxes are nice, for more then just additional speed. I've never purchased a dedicated "formatted" seedbox per say, but I have purchased raw dedicated servers on many occasions. Doing that you have the freedom to install whatever operating system you want, whatever torrent client you wish, and any other applications you might need. For example, handbrake or some encoding app on an offsite box might be one use case among many. They are nice, but they come at a premium price point compared to shared boxes in which people on a budget might find it hard to justify.