no matter how good the manufacturers make their cars, someone will feel the need to mod them. so why not make them at the lowest cost, and then sale the after market 'improvement' parts? make more this way.
a 18'' spare weight about 50 lbs already, so it is easy to lost a couple hundred lbs, for mine, the couple weight about 500 lbs ( I think..) less than the soft top, and the base version weight even less (I think.. again).