Split crown races are easier to install but might not stay put as securely and if allowed to move about (loose headset adjustment as example) might result in wear/damage to the crown's seat. I take the split race thing to be about working around poor dimensional control of the crown seat during the fork's manufacturing. With a variable race ID the crown seat can be off dimension and have less headset adjustment issues.

BTW if I saw someone using a hacksaw to mill down a crown seat I would run in the other direction Andy