My 2013 came with the front strut tower bar. I don't know if that is because it is the premium level (MCA edition) or if all convertibles have it. Supposedly it tightens things up front, but I really didn't notice it being that much different from my 2006 convertible.
A strut tower bar helps to maintain suspension geometry (Camber) in hard cornering by adding additional support to the top of the Strut towers that often flex under sharp turns or higher speed manuevers.
This will help improve handling characteristics by keeping the contact patch of the tire where it belongs, on the pavement (planted). This improves handling especially at the limit but also has the downside that it can make that limit more black and white.
Why is that a downside? The average driver is more likely to lose control with a very taught responsive suspension and steering system vs one that is forgiving (sloppy) at the limit and allows for a greater margin of error. Granted, a bar won't make that profound of a difference by itself but every bit counts.