I don't think dismemberment is inherently a gimmick. I think realistic implementation is key. Limbs should only dismember if the correct weapon and proper force is applied at the correct angle, which may or may not take successive strikes. This would be very rare. Wearing almost any kind of armor would make it even MORE rare.
The reasoning for implementation is to deepen the games immersion factor. Especially if dismemberment is not canned animations but very unique to the strike location (for example, some games that include dismemberment, the limb always gets ripped away at X location. If, rather, the dismemberment occurred exactly where you landed your blows, this would create a very dynamic implementation).
There's other arguments in regards to the necessity of implementing dismemberment. It is a very gruesome thing, and when you take into account real life examples of dismemberment, it seems--to a degree--a bit brutal to implement in a game. But should we shy away from the reality of this type of combat (swords, axes, etc.) and how the body reacts to receiving blows, no matter how rare.
To me, the world, environment, animations, and the physics, etc., is key to telling a story. When you tell a story, especially one that involves fantastical elements (e.g, thaumaturgy), it is important for all these other elements to be rooted in realism. It creates a more immersive world and makes everything else more realistic, more believable.