I suspect this practice is old (maybe from the ages roles weren't invented yet) and grown into the compagny way of doing things.
Change always creates resistance. One off the problems would be learning how to use and restructure into a role based security.
This can imply several weeks of lower productivity so especcially if some employees sabotage the change, this can happen if personal emotions are involved in the decision to change process or there are already tensions between groups of employees or permissions reflect some kind of status in the compagny.

All in all it can be a simple change but it can be a disaster as well.