I would suggest that the newer lightweight lathes tend to have rotating headstocks, but then they don't usually have a threaded spindle on both sides of the headstock, either (Jet, Delta, Etc) Most bigger lathes do have twin spindles, one inboard (RH), and one outboard (LH). Some medium to large commercial lathes have only one threaded spindle nose, but the headstock slides all the way to the end of the bed for non-reversed outboard turning.
It is probably wise to get the dual-threaded insert for your chuck if you think bigger pieces and outboard turning are in your future, or, if your lathe doesn't support that method, if you can envision taking your chuck to someone else's lathe to turn a big bowl outboard. (For example I can only turn 12" inboard on my lathe, and used my equipment to turn a 15" piece on the outboard side of my dad's lathe.) The only drawback seems to be how easily the chuck will thread on to the spindle -- some report a dedicated RH thread is easier to get on straight with less fussing. I use a dual-thread chuck insert whenever I turn in my dad's shop, and I'm not sure I ever noticed the difference.
The only stupid question is the one that you didn't ask, and an injury results ...
Safe Spinning,
Brad Vietje Thetford, VT