Umm....

1. Ask all you want, that's part of the reason this place is here...perhaps the main reason
2. Maybe they feel that using 150gr bullets is a waste of the 300magnum's potential OR maybe they feel that they don't hold up when they're pushed at 300 velocities. If that's the case go with a partitioned bullet or barnes.
3. Lighter bullets are generally shorter than there heavier partners. Shorter bullet lengths require slower twist rates to stabilize. Longer bullets require faster twist rates to stabilize. I highly doubt that .30 caliber 150gr bullets are going to have problems "flying apart" from too fast twist-rates.