I was lucky enough to "win" a premium pinata for a plant i actually have at the pinata party which took me over the experience points needed to level up and the 10,000 coinz are the price i have to pay to EA to actually level up.
Don't ask me what they spend them on.
Those 5,000 coin pinatas are not actually pinatas but a daily offer put out by EA to get you to visit the shop, watch ads or buy coinz.
Hurrikale is the first gemium plant i bought as soon as i had the 100 gems and i occasionally get a premium pinata with seed packets for it.
I soon found out after buying it that every time you lose it costs you an extra 10 gemz to not to lose your streak when you lose a battlez match (10,20,30,40 and so on).
It is well worth saving them (and mintz) for a rainy day.
Most of the prEAmium pinatas i get are for plants i haven't bought so if i do buy one then i usually can level up 2 or 3 times.
Now the rewards for a successful streak are BIG by my standards (the furthest i got was 44) but i woke up at 3am and decided to use a freeplay without setting up a defence before starting (DUH!) and it would have cost me 70 gems to retain it so i declined.
ThEAy usually push premium pinatas for a new plant ahead of said plant appearing as plant of the week.