Rule of thumb when comparing either HP IV or defense IV should be higher for certain pokemon.
I have read through this post in chinese and it has the answer to the question above. http://www.pokemon.name/thread-448989-1-1.html
The rule of thumb is: if the pokemon has higher base defense, you need it to have higher IV of HP; if the pokemon has higher stamina, you need it to have higher IV of defense.
Answers
You don't 'need' it per-se, I have 2 Vaporeon at 15/04/15 and 15/15/04 and the one with 4 defense (and it is Vaporeon's weakest stat) is the one I prefer because it allows you to take more damage (while still maintaining the HP to soak it up) and use your charge attacks.
On paper, yes, your reasoning makes sense (i.e. my 15/15/4 has 2651 CP while the 15/4/15 has 2627) but I'd almost always prefer the HP IV regardless of what base stats were there to begin with, unless its an extreme case like Cloyster.
