1.They offer us some insight on the Pokemon World in an interesting way at times.
2. Now and then, they have nice personalities along with their Pokemon like Katharine and her Gothita.
1. They often waste time that COULD have been used to develop our main characters (you know, the ones we're meant to root for and follow) and maybe an overarching storyline.
2. Their episodes follow a formula that I think doesn't bear repeating since everyone here practically knows it by heart. Even with slight variations, it become repetitive like in Johto's dreaded filler hell or DP with a year without badges.
As you have said they can be interesting. It depends who is writing the episode really, because I have found some interesting CoTD and then I have some some generic ones that are just there to waste space and are super forgettable. I want my CoTD to be memorable like Charles in Driftveil City. I found him entertaining, funny, and action packed. Some on the other hand just follow a generic recycled plot so there is really nothing to them to begin with expect to fill a episode and bide time. At some point the CoTD along with there episodes just get tiring and leave me wondering when they are actually going to jump back on the main plot.