I'll come with

Why do some people (usually from a particular part of the country) say "I'll come with" instead of "I"ll come with you"?

A particular part of which country? I'm probably wrong to think of it as an Americanism, and I think I'd prefer to translate it as "I'll come along" rather than "I'll come with you," accentuated "I'll come WITH you." I think it's probably just an example of the American taste for ellipsis and whimsy, and I think it's a relatively recent locution (last century). It could have some connection with the German use of mitkommen, which means to come along. Darf ich mitkommen? Ja, komm mit. SS

My grandmother, who was a first-generation American of Danish/German ancestry born in 1903, used to say "come with", "go with", etc. She lived her entire life in New Jersey. I don't know if this helps the original poster, but it seems to fit Smokey's German theory.

I am in the U.S. (West Virginia/Kentucky). This expression -- come with -- is fairly new to me.