Rarity's last episode before Season 4 was Episode 35, Sweet and Elite. Her last shining moment (not shared with the other Mane 5) is in the following episode, Secret of My Excess, where she returns Spike to his original form.
It's also the only episode where they both show each other affection. For the first two seasons Spike's crush on Rarity is clear, but whenever Rarity shows any affection to Spike, especially in Dragon Quest, Spike doesn't return it. He completely ignores her. Is he over her now or what? The only episodes where he seems to feel nothing for her are the ones where she loves him the most.
2.) Lack of episodes
Although she finally gets some more episodes in Season 4, it still stands that for over half of Season 2, and all of Season 3, Rarity has no episodes of her own.
3.) The comics
This doesn't come from the show's writers, but in the comics, Rarity doesn't share the adventure with the other Mane 5 this time. Why? Because she's possessed by Nightmare Moon, who kidnaps her and takes advantage of her insecurity to get her to actually allow her to possess her. And there's that one scene in Issue 8. All of this has led to someone saying Rarity got the "(censored) slap of her life". (Can we say that word? I'd rather not take the chance, even though there is a nickname for Fluttershy's discorded form) And I agree. That arc is definitely non-canon, mostly due to Princess Twilight Sparkle, but still.
4.) Doing to her what can't be done to Tirek
I thought I was over "Rarity doesn't get enough love" when she finally got some episodes in Season 4. But then I came to the realization - Rarity is the only speaking character in the show who has been punched in the face. (only other character to be punched in the face is a changeling, and his attacker ironically is Rarity)
Now, they said they couldn't show Twilight punch Tirek in the face in Twilight's Kingdom. Why? Because, apparently, punching a speaking character in the face is taboo for a show that younger viewers can watch, the only family-unfriendly violence that doesn't involve blood and gore. We've seen speaking characters get kicked, smacked, and tail whipped in the face. We've seen heavy objects fall on Twilight. We've seen Twilight violently thrown into a mountain and smashed through it. We've seen Tirek violently smashed to the ground. We've seen Rainbow Dash's wings get broken or damaged countless times. We've seen Discord cut his own head off. We've seen Rarity cut off her tail, which is like a limb to her as it is later shown to be prehensile. We've seen characters die. We've seen characters go to hell. None of that is too violent for MLP, but punching a speaking character in the face is.
Yet in Princess Twilight Sparkle Part 1, Rarity gets punched in the face by a Plunderseed. So Tirek can't be punched in the face by someone whose home he destroyed, but Rarity can be punched in the face by a villain? You might argue that Plunderseeds don't have fists so they can't "punch", well guess what, ponies technically don't have fists either, they have hooves, yet they can punch with them. The only logical explanation is that they don't like Rarity enough. They hate her so much they allow violence to be used on her that can't even be used on the second most evil villain of MLP.
Come to think of it, she's the only character whose blood we see onscreen, aside from Twilight receiving a scratch. In Magic Duel, Rarity pricks her hoof with a needle and we see blood on it. Blood in MLP. They didn't even make her blood silver (which would serve as a reference to Harry Potter as well as making the violence more family-friendly, like in Sailor Moon when they made Neflite's blood green or Stardust when they made Primus's blood blue (blueblood lol).) I'm all for ways of saying "This isn't some little kid show" but is Rarity the only character they can do that to?
Maybe they saw the light after, before the Rarity episodes? Well, the writers gave her new episodes, but Hasbro makes the call for what they can and can't do. I'm surprised Hasbro even approved Rarity getting any episodes in Season 4 if they hate her that much they'd make an exception to their rule about punching speaking characters in the face. Well, I suppose Rarity Takes Manehattan was necessary for Rarity to get her key. (yet apparently they don't hate her enough to not make any products of her, unlike the supposed background pony Applejack... Hasbro's marketing crew is monkeys, or I'm one of their uncles. For once this might be a good thing or there'd be less Rarity merchandise)
Everypony, give Rarity your love. She needs it right now. She doesn't get a lot of it from the creators.