Better cover penetration, more immediate incapacitation, versatility of range, better for hunting if necessary...and battlefields have indeed changed. If for whatever some fantasy siege of whatever entity was attacking, it may happen in an urban environment with lots of cover, cars, light buildings, etc. - think Iraq. Even rurally there is plenty to hide behind. And a .223 just doesn't cut it in a lot of cases. Bottom line, it's harder to hide from a .308 than a .223. Those "overweight, slower handling weapons" such as the FAL seem to be just fine for the over 90 countries that have employed them as their main service rifle.