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Have I ever mentioned that I like Mirri, Cat Warrior? I do. I also like to rancor Mirri, Cat Warrior. It makes me happy. You may be saying 'duh, any creature is good with rancor' but give me a minute to explain. Is River Boa good? Yes. Why? Because it has multiple useful abilities for a reasonable cost. Mirri is in the same boat. But that's not all! I like Mirri specifically in the extended environment. I said this last season, and the reasons behind it are even more valid now. With the new extended trying to look towards more creature based decks, it is the high quality creatures that will rise to the top. And when you look at what deck types are still playable, and, in several cases, all the stronger for the recent bannings, you find she is well suited to wreak havoc amongst the archetypal players.

Let's look at Mirri and just a few of the decks which (perhaps only in my humble opinion) are strong tier one decks for the new extended. Lockdown style decks: (brothers and sisters of stasis… with intruder alarm, static orb. Etc.) there always has to be control in the environment, and I see these as something which may crop up again this coming season. Mirri has her spiffy 'doesn't tap to attack' skill that could make her an invaluable tool against these annoyances.
Beatdown Decks: (sligh/red weenie, stompy, etc.) There will always be speed beatdown decks, and Mirri has got the talents to help you against them. Her serra-like self gives them a bruising while leaving herself untapped for defense; her 3 toughness and first strike abilities facilitate that… against sligh she stops ball lightnings cold and can't be shocked, while against stompy she can't even be blocked. The first strike/3 toughness is good protection against a wide range of small critters on offense and defense. She can block longbow archers and come back for more, waltz past multiple jackal pups without worries, etc. 
Oath of Druids: one of the strongest deck types left. She forest walks. Keep the oath off the board, keep her alive. What? Forest walk is a strong ability. I didn't say she was non stop beats against everything by herself. That's when we come to our next topic-

RANCOR!
Suddenly she does it all. Blocks and attacks against masticores. Becomes a threat against control decks. Tramples, first strikes, generally wrecks other creatures in combat.
Attacking and defending with 4 points of first strike all in one creature is not anything to laugh at. A rancored Mirri should be a threat to almost any deck (a bigger threat than a normal rancored creature). And that is why you can play 4 of this legend. Like Lin Sivvi, she has the potential to be such a threat against several decks that she becomes a 'must kill' creature. Having the redundancy is a good plan. And let's not get in to her potential with multiple rancors… 
Protect Mirri, and she will protect you. You may disagree with me or agree, and I am not saying she is THE powerhouse of the environment; I would just like to point out that if someone can find the right deck to put her in… suddenly she becomes a 10 dollar rare.
Good luck and have fun.