They're definitely not cost effective and certainly not meant for everyone. Even still, there are scores of people out there with large excesses of Justice Points, with no need (or ability) to obtain more, and nothing to spend them on. This provides an excellent alternative for a select portion of players.

For everyone else who isn't concerned with the Justice Point cap or how to spend them, these vendors can be disregarded. They're not meant to replace the activity of farming for tradeskill materials for the general populace.