Why would you overlook a stronger rider just because you run a NL/NT team?
If every club chose replacements from their development team only, then clubs that don't run a development team, would be at an advantage, being able to pick from any NL/NT team for a replacement.
By that logic every Prem team losing a RS would be signing a NL/NT rider as a replacement, while Ipswich could go for Gilkes or Hagon because they don't have a NL/NT team to recruit from.
Also the NL/NT is miles away from the Prem, even at RS level. It's better riders move up when they are ready & not before. If one of the top NL/NT riders Max James, steps down from Edinburgh because it's too tough for him, surely you wouldn't put him in a RS spot just because he rides for Birmingham's second team?