I'm more or less on No Shos side with this as this isn't about one trainer, it's about every single character in the mix. Allowing them to catch certain pokemon because they reserved it takes away from some of the development of characters since oocly they know they are going to get it some time.

Plus I have a little example for why reserving them shouldn't be a thing.

Lets say hello to Terry and Jerry, both are trainers in the rp. Terry didn't reserve any legendaries, however Jerry did. Terry is up against a long gruelling battle against said legendary and when the time comes to catch it...

1. Jerry ninjas it from him with another pokeball, making hard work hardly work. Sure, it can lead to some character development, but it depends on what the characters are like.
2. Terry catches it, but Jerry takes it since he reserved it, making it the same situation as before in a way.

Legendaries are called legendaries for a reason, so in my opinion, we shouldn't meddle with this until later game in a plot. Temporarily having them join you for plot reasons I can sort of see, but permanently? Kind of a stretch in my opinion... And it may make problems in the future. My two cents on this spiel.