This rule is a backward step in my opinion.
Looking back on last season the amount of riders injured actually seemed to increase and there will be times where teams will struggle to field a decent side.
If an away side have either their second or heat leader out injured then they are not going to be as competitive using rider replacement instead of a guest.
This will lead to more home wins and in my opinion will have an adverse effect on crowds as many neutrals will not bother going to meetings as the away side will be weakened considerably.
A more sensible approach would have been to allow guests for heat leaders only with a riders home or away average used to decide the level of guest allowed.
This would stop track specialists being used as guests for riders with poor away averages.