[sharingan09]

That invincibility like you stated could be true or that just, because he has such a great skill as a ninja on his own he is really strong, and having another 6 bodies with the same level of skill as the original would be tough to deal with... so technically hes "invincible"

But I agree as to why he would be taking orders from Madara when he could probably beat the crap out of him.. unless theres something hidden that we dont know about Madaras powers like how he can become hollow and let attacks just go through him..