I think the best part is people hate on OCs so much and then go and assign an arbitrary personality to a background pony, write about her/him and don't even bat an eye.
For writing, my philosophy is the story goes where the story needs to. If the role can be filled by one of the main six or a secondary character with a semi-decent amount of screen time, I use her/him. If there's a contextual reason to use background ponies (like say, the spa sisters because there's a scene in the spa), then I'll use them. Otherwise, if I'm going to basically going to be grabbing a random colour scheme as seen for 5 seconds on the show and assigning an arbitrary personality, I might as well use an OC. For example, at the start of Evil's Twilight, I needed a peg who was a fairly good flier to come tell Twilight just how much the storm was screwing over the weather team. The Wonderbolts would make zero sense, so they were out. Fluttershy is animal team, so she was also out. Dashie was out because I needed a peg holding back the storm and we all know she's at least the best flier in Ponyville. So it came down to either background pony or my own ponysona/OC. My pony had the benefit of a personality I had already started to think through, so the choice was a no brainer, really.
Oh and I've also had a few specialized roles that none of the existing ponies could really fit, so that called for custom tailoring an OC.
As far as reading goes, I'm personally fine with any well written OC.