Are you
A Nut
I’m a buddy to some, but a wary adversary to others.
Fitting a nut can be easier for a few but others take a precaution.
Yet I can also be a protector to the weak.
It keeps the weaker and falling things intact
You can even sometimes see me with my friend, Jack.
Jack Nut is a kind of nut.
Crazy guys take my name and spout it about.
Nut's is a slang.
Some would take it as an insult but, I don’t.
Saying "nuts" to someone might be an insult to a person but well, for nut its not.
Original poster's thoughts
I accepted @Techidiot answer because I believe it was the closest to my original answer as possible.
Original Answer:
a peanut. Techidiot answer was close enough.
I’m a buddy to some, but a wary adversary to others.
Peanut butter is known as the buddy of jelly or jam. Yet some are allergic to peanuts, so they have to be wary of them.
Yet I can also be a protector to the weak.
Packaging peanuts or just called peanuts, are used to protect fragile items when shipped.
You can even sometimes see me with my friend, Jack.
Take me out to the ball game. Buy me some peanuts and Cracker Jacks.
Jack of Cracker Jacks has a dog named Bingo.
Crazy guys take my name and spout it about.
Nut guys yell "Peanuts! Get your peanuts here!" at sporting events to try to get your attention to buy some.
Some would take it as an insult but, I don’t.
The term peanut refers to calling someone of short stature as an insult to them.