For 5mans, this seems to assume that frost death knights are tanks (when frost is also a very good dps spec), rather than drawing from the auto-group feature that indicated what roles people play. Well geared ret paladins also often tank heroics as ret, and this automatically lists them as DPS - again instead of drawing from their assigned roles in the auto-group feature.
GridStatusRole uses talent scanning to determine role via LibGroupTalents-1.0, and it predates the Blizzard LFG role system. However, it works whether or not you've used the dungeon finder, it works in raids, and it works in BGs. It also works if party members change roles during a session.
Death Knights are considered tanks if they have points in at least 2 of Blade Barrier, Toughness, and Anticipation. Warriors are considered tanks if they are specced primarily into Protection. Paladins are considered tanks if they are specced primarily into Protection. Druids are considered tanks if they are specced primarily into Feral and have points in Survival of the Fittest and Protector of the Pack.
Hunters are listed as "Melee", which I generally hope the hunters in my groups don't do.
I would suggest categorizing DPS roles as Melee and Ranged, rather than Melee and Caster.
Death Knights are considered tanks if they have points in at least 2 of Blade Barrier, Toughness, and Anticipation.
Warriors are considered tanks if they are specced primarily into Protection.
Paladins are considered tanks if they are specced primarily into Protection.
Druids are considered tanks if they are specced primarily into Feral and have points in Survival of the Fittest and Protector of the Pack.
This will only work in random dungeons and won't update if you change roles in the instance.