I think that allowing new players to tweak their characters and have their build solidified by their fourth mission is an important part of retaining them. If they make a character that they like conceptually, but find that they totally screwed themselves on a certain quality or allocation of points and we just stand firm with arms crossed and go "Too bad", then we're not really doing our jobs to sell the game.
RPGs are meant for groups of us to have fun and enjoy ourselves. If something a request reasonable as, "Oh, hey, I thought Quality X worked like this and it totally doesn't, can I change it, seeing as I've only played a single mission with this character" is just 'shut down', then the player might feel shut down and be chased away from the table.
So, I'm for this and would be surprised if this wasn't just happening, in general. But there definitely needs to be a firm line and just watch out for abuse.