I don't think the argument is that NC presenters aren't senior enough, it's that a different team produces the bulletins and the presenter is part of that team, (re)writes the script etc. They can't do that if they are on the air on the News Channel.
Yes - it's important not to merge two issues here :
1. Is a different/second presenter required to present the longer BBC One bulletins (to allow for pre-transmission prep work)?
2. Should that different/second presenter be a 'BBC One' face or can they be a 'BBC News Channel' face?
1. The early evening bulletin on a Saturday is ten minutes in length when the schedulers are being generous (often 5) which is no longer than the lunchtime bulletin. The duty NC presenter is off air from 20:30 to 22:00 which should be more than enough time to prepare for what is usually an 18minute bulletin including a 5 minute sports update from Salford that the presenter has nothing to do with.
2. Realistically it could be the breakfast newsreader on radio Sheffield as the weekend bulletins contain very few lives and none with anyone external to the BBC.