You are right. I do believe Madara understands the effects of the full Susano'o, and understands it's both too strenuous and hinders mobility.They would be relevant if you said itachi's susanoo> than any other jutsu, than that would change the debate considerably. Instead you are trying to say that susanoo the jutsu itself is the most useful. What you fail to realise is susanoo the jutsu itself, does not come with the sword and mirror, and that is why you point is irrelevant for this discussion. Also since it obviously need to be stated again, susanoo is chakra based, so is sasuke's sword/arrow. If kakashi has the time to activate a kumai to defend against an incoming arrow nagato could easily get the chakra absorbing shield up. You have again demonstrated how simple minded you are.....
Apart from Madara (or anyone for that matter) with an incomplete susanoo, we have never seen someone that is very mobile with a complete susanoo up. Your argument seems to center on a complete susanoo this is why I made the point. Again stop and think before you post.