I sometimes wonder this myself. When you think about it, however, 14 miles isn't that long, and the scheduled time between the two stations is only about 15 minutes. I think the building of the South Brunswick/Monmouth Jct. station will be corrolated along with the building of the MOM line, as that is where trains are rumoured to terminate.
Also, I wouldn't expect a full station at Jersey Av. anytime soon. One thing I don't get about Jersey Av., however, is why they can't just build another low platform with a cheap crossover on the northbound track. It doesn't make sense. Also, if you've ever been to Jersey Av. on a weekday, the large parking lot pretty much fills up, even though the station only sees about 6-7 trains in each direction each day (actually, I think it's more than that now).
About 5 years ago, I lived at my father's house in Somerset for about 5 months after just getting out of college, and worked in the city. I commuted from Jersey Avenue, and although I don't remember much, I remember alot of people boarding the train there.