The problem is that you have to have a DM buy into those roles.
Our DM and many others based on feedback on these forums don't. They will ignore the tank, even taking opportunity attacks to "geek the mage."
If you are a DM that abides by the standard fantasy genre where the fighter stands toe to toe with the enemy while the mage casts spells in the back then I applaud you. Unfortunately that seems rare these days.
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So ... it's a bad thing if DMs run NPCs intelligently using tactics they believe the enemy would realistically use?