You're writing as you go, and you have to scripts ready to shoot, so at some point, the question of "Is this character a good idea?" becomes moot, and you just have to finish a script and film it. (Kristen could say way more about this than I can, since I'm doing a lot of assuming.)
Late to the party. Writing for TV is like trying to hit a moving target. (I'm pretty sure someone else said that first but it's true.) And, as the season goes on, the target moves faster and faster.
My first episode of last season, I spent six weeks on the story and the outline and I think I had almost three weeks to write my script. End of the season? From initial concept to script in less than two weeks. The draft was written in six days. (There were other episodes where multiple writers had to pitch in because there were only 3 or 4 days to write the script.)
So that's the first part. The second part is this:
You don't have the luxury of looking at the finished season and saying, "You know what, this isn't really going where we wanted it to, so let's scrap it."
So much this. We start out every season with a plan. We know our endpoint and some of the big tent pole stops along the way. But then you start production and shit happens. You have a great character but you can't get the actor you want or you get the actor you want and they suck or you get the actor you want and they're great but when you try to bring them back, they're not available. Stories that seemed great in the room turn out to be a giant yawn on screen. Etc.
So you're forced to improvise along the way. But that's not always a bad thing. One of our best episodes of this past season was an improvisation. As we were breaking the episode, we found out we weren't going to be able to do the story we had planned. So we threw it out and, in two days, came up with a new story.
There are so many times you wish you could go back and fix things but it's the way of television. You have to keep the plates spinning. With all that said, there are many reasons why a certain storyline or character wouldn't be included in the recap. The most obvious reason is time. You get 42 minutes -- including your recap. Every second you use for that recap is one less second you get in your actual episode. So you cut anything you consider non-essential to the story you're about to tell. You cut and splice (and sometimes even revoice dialogue) to make your recaps as short and to the point as you can.
There's also the reality that, if you hated the way a story turned out, you just want to cut your losses and move on. I have personally taken that position once or twice.