First of all,
flip Jace doesn't have a -2. His ability costs 3 loyalty.I don't think a PW should earn its full mana cost the turn you play it. They generate a lot of advantage over time.
I also don't think that a PW should necessarily be auto include if you can afford to cast them. Some exceptions should be made for mana intensive or expensive PW like Tamiyo or Sorin.
Lili is the cheapest non-flip PW we have and her abilities are quite comparable to flip Jace. Jace finds play in many, many decks in standard.
Lili can:
+1 lock down a bear or kill off a small creature, making edict effects (such as her oath) much more powerful
-2 mill you for two and bring back
any creature in your GY, not just one of the two cards milled.
Just imagine using her abilities in conjunction with
Fleshbag Marauder. +1 you take out sac fodder, -2 you force another edict effect.
To make the comparison with flip Jace again, his -3 ability is quite comparable, but it costs 50% more loyalty and does not self mill.
Lili is not worth the entire mana over flip Jace, but her first two abilities are more powerful in the right deck. Flip Jace's +2 loyalty advantage gives him an advantage early on but eventually Lili can activate her - ability more frequently.
It is funny that people are very excited about the new emerge mechanic and creatures that provide some benefit upon sacrifice such as
Deadbridge Shaman. Lili can allow you to recur the whole mess and do it over again.
Again she is boring, and not the most powerful PW ever, but I think she is playable and her power level is appropriate. Sometimes, every once in a while, Wizards R & D gets it right.