Nobody in their right mind "wants" to be an INFJ, if somebody is wanting to be one it's only because they've never had to live as one or they'd be wanting to be an INFP. If I had to choose between those two, I would much rather be an INFP!!! I'm sure most INFJs in fact would rather be an INFP.
Firstly, I don't think INFJs are as rare as we think.
Secondly, I think they're the most commonly mistyped.
Thirdly, I think they're the type most likely to be unhealthy. In other words, it's harder to find an example of a healthy INFJ than any other type (percentage wise to account for their "rareness").
Lastly, INFJs are some of the most emotionally reactive, vindictive, evil little things I've ever met, they're prone to self sabotage, martyrdom, intensely manipulative and self deceived about it and the list goes on. The online saint like descriptions seem to only feed that perception of themselves and make the entire issue worse.
A healthy INFJ is a sight to behold, they're empathetic, upfront, truthful, loyal, flexible, comfortable in their own skin, dynamic, outspoken, motivated, and really spectacular people. I however can count the number of healthy INFJs I know on one hand and don't need the thumb. Most of them "know" what that looks like so they can talk the talk but simply can't walk the walk. Anybody who can't see how Hitler could have been an INFJ has simply never met an INFJ beyond their persona. Anybody who has truly met an INFJ can also see how Jesus, Gandhi, Mother T and MLK were the epitome and embodiment of a healthy INFJ.