I get that people usually love polygamous relationships, threesomes, harems, Herds, whatever you want to call them, as long as they're between the mane six because friendship with magic benefits between the mane six is super cute... until an OC is introduced. The second an OC enters the mix, it's immediately a stupid harem show cliche-storm with a gary stu protagonist, straight out of the kind of stupid uninspired trash anime every real anime fan despises, because it means some corporate hack is funding a studio that's producing this kind of trash instead of Gurren Lagann 2: Spiral Harder.
Why do people hate Herds, when used in fanfics? While I hate harem "love at first sight" "Collect all the mares in order like they're six crystal shards of a magic door" "Dash punches him but loves him anyway and he loves her because pretty rainbow tail" "Spike either magically loses his crush on Rarity, dates Sweetie Belle, dates Raincloud, dates Cloud Kicker, or hates anon/OC forever" nonsense, I do like shipfics, and I feel harems/herds can be done right, much like OCs and fanfics in general.
Plus, on a more emotional note, there's the fact that herds involve the characters getting together for an adorable loving snuggle-pile, and that's cute.