It makes no difference.
It makes a pretty large difference. Displaying the class/job is informative and always accurate (Um, a WAR is always a WAR rather they're tanking or not), wheras displaying pre-assigned "T"s or "A"s is informative but not always accurate. Having a A1 next to my main tank is not helpful in the slightest and only made things unclear to new players. If you can't remember the classes/jobs in this game and their roles in combat, there's a problem. T's, A's, etc. will only be useful if they are changeable by the party leader.

Well maybe they get a choice ?
Hello. My name is Michael and this is what I've been asking for the whole time. -_- Thanks. But in some situations, some jobs could play roles outside their "normal" role, and some normally do play multiple roles. So instead of restricting choice to MRD/WAR, why not give everyone the choice. -_-'''