Would you join a VA specifically intended for offline flying?
I don't think being specifically intended for off-line flight is that great of an idea. Airlines that have the option to fly off line is a great convinience (i.e. UPS virtual, UVA virtual, ATA virtual, etc.), especially if you're new to on-line flight or don't have the time to be at your computer the full length of a flight. However...
When you fly on VATSIM or IVAO, you certainly enhance your realism many times over, develop a strong bond / sense of community with your fellow pilots during social events like fly-ins, and have the chance to learn far more about flight in much shorter time.
In short, I'm not much of a fan of airlines that limit the person to either one or the other.