When I first played the game to be honest, I mindlessly said yes to everybody who asked. I didn't care really if they called me, in fact it was rather interesting listening to them talk about various things in their life as a Pokemon trainer. To learn more about the people in the game, to get different information each time rather than always get the same response, I actually enjoy calling certain trainers from time to time myself. While the advice they give is awful, it's still nice having a chat with them. Also I thought it was a neat feature and it couldn't hurt to say no, so basically I registered nearly every trainer there was simply because I was to lazy to reject and the reasons above. XD
Now I still think it's the same really. I reject maybe one or two trainers but other than that hardly at all.