It's rather clear the amount you can strengthen yourself is tied to which school you are. Observe
http://www.mangafox.com/manga/hunter...09/c083/3.html
If anybody could just use their full aura to defend himself, why is Ubogin unable to tell which school Kurapika came from? At one point, he actually thought Kurapika could be from the Strengthen school.
It's not that people outside the Strengthen school can't use it, but if you're say, Manipulation or Materialization, your ability to use such abilities would be a joke compared to someone who is naturally that type.
And go to this
http://www.mangafox.com/manga/hunter...10/c084/9.html
Kurapika referred to the fact that he can overpower Ubogin's body as 'precious information'. If it is clear that anybody using aura can beat someone who is not, why on earth is this valuable information? Kurapika clearly thought it could be possible that someone's physical body alone can be strong enough to withstand his aura punch. This also reveals that the physical difference between him and Ubogin is actually not very big.
Even Kurapika, who has Absolute Time to help him bridge the gap, he was never able to do effective damage to Ubogin. A manipulation user would have the exact same issue, but assuming Absolute Time isn't something every random guy could have, they'd face an even bigger hurdle (1.36 versus 1.60). I guess you can say maybe someone can manipulate someone without ever needing to do something that potentially could cause damage, but if you go by that then you could say what if Gensuru can blow you up by just saying "Bomb Devil". There are a whole mess of abilities that'd be utterly unstoppable if there are no restrictions to them.