There is actually nothing stopping Myrmidon from being an offtank, or even main tank in some situations and that is exactly what this stance is for. Gaining free defense with no drawback (reduction of attack) would probably make the stance too strong.

That being said, it's not a straightup 20% reduction in attack - as with all percentage addition/reduction it is only a percentage of attack gained from main stats such as Strength and Agility, not from Attack Power. Myrmidon gear doesn't really focus on Strength items so you wouldn't be taking a big hit on either attack of defense when using this stance tbh.