It is a stupid argument, yes, because different rules should apply to different things. Realism, which is the game being consistent with itself, is great to be applied when possible, but not when it tramples gameplay.
It is silly to say that, because x is unrealistic anything can be unrealistic, because the game is already trying to be realistic. But it's also silly to say that the game should be 100% realistic when it comes to certain things like weight, because that would kill building. In this example a good inventory/slot system is the best of both worlds, giving off some realism/consistency and allowing unhinged building.