Really depends on the way YOU look at it. I live in NJ as well, so as you know, I do see a ton of Z3's and Z4's running around.
A Z is perfectly fine to use year-round as long as you get snow tires for our NJ winters. I'd say the only downsides come from the soft top, and here they are.
1.) Soft top does not retain heat as well as a hard top (such as when you're driving around during the cold months, pumping the heat up).
2.) Soft top will experience more wear-and-tear when it is subject to harsh conditions (very cold weather, rain, snow, etc.).
3.) Soft top as a daily driver is fine for the driver, but to everyone else, it just looks "out of its element" to see that ragtop putting around in sub-40 degree weather.