"If Christians aren't Mormons, why is it OK for Mormons to want to be called Christians?"
My response was:
Let me re-phrase your question in terms of another set and sub-set - one that has direct and obvious historical precedent:
"If humans aren't African-American, why is it OK for African-Americans to want to be called humans?"
This is a simple question of sets and sub-sets. It's not any more complicated than that.
If it's OK for people to disagree about some doctrines (and Protestants of different denominations have disagreed VIOLENTLY over lots of "foundational" doctrines over the centuries) and still be considered Christians - and if the only immutable concept which everyone within Protestantism must confess in order to be considered Christian is that Jesus is the Christ and Savior and Redeemer (which, in a nutshell, is the standard for Protestants who disagree vehemently about lots of other things) - and if, in fact, Mormons actually do confess this required concept (which, in fact, they do) - then the only way to exclude them from being Christian is to change the rules explicitly for them - to hold them to a different standard and definition than that to which others are held.
That's hypocrisy, pure and simple - since, just as African-Americans are humans no matter how others might have excluded them in the past from that category, Mormons are Christian no matter how others still exclude them from that category. It's a simple concept made complex by people who simply are unwilling to let go of the incorrect biases and prejudices of the past, just like White Supremists who still won't admit that African-Americans really are fully human.