It's true the prime advantage is weight, particularly with the bulky and heavy low speed generators for gearless turbines (and to a lesser degree hybrid turbines with a one or two stage gearbox.) net energy loss at this point is minor, perhaps less than a percent.

Saving weight is about lessening loads on rotating equipment, and lighter generators help.  perm mag gens when properly designed likely decrease downtime and mtbf.

There are likely to be other sources of rare earths down the road, but that's just speculation on my part.

"Life shrinks or expands in proportion to one's courage." - Anaïs Nin