My unofficial rationale behind the "sails" is that they were a pseudo-experimental modification to the standard FTL drive system to extend its range and accuracy. In essence, they capture and focus the spatial distortion from the drive spheres, enabling it to jump much further than other ships in its class and much more accurately.
Unfortunately, there are two downsides to the technology:
1. The sails are rather fragile and vulnerable to misalignment, potentially resulting in wildly inaccurate FTL jumps.
2. The sails are virtually opaque to radiant energy, trapping heat in the power coils surrounding the saucer. This is why the sails are left partially open much of the time.
I'm thinking the sails were one of those "good idea at the time" kind of things that came and went fairly quickly in the evolution of ship design. They are still, pound-for-pound, the longest range, most accurate FTL drive systems ever built, but too high-maintenance to justify on most designs unless you really need those capabilities.