Pinkie Pie is worth employing · 2:29pm Apr 23rd, 2017
❧Pinkie creates a lot of business for the Cakes. Early on, at least, many parties were just plain at Sugarcube Corner.
❧Pinkie creates a lot of advertisement and networking for the Cakes. (Rarity aspires to be the pony everypony should know. Pinkie…knows everypony in Ponyville. I expect everyone is on a first-name basis with Pinkie, too, and that's a big influence for business.)
❧Pinkie's parties are (maybe) not funded by her: She's the premier party planner of Ponyville, not he party host.
❧Pinkie Pie can set up or clean up a party in seconds (S7E1).
❧Pinkie PIe does deliveries (and can outspeed Rainbow Dash, or get to YakYakistan and bak, er, back in a day).
❧Pinkie is a world-renowned baker. Okay, no, the Cakes made the MMMM, but she helped make the combination entry that won…and is known overseas (in Griffonstone) for baking.
❧Pinkie Pie can invent and build flying machines, is an exotic dancer, either of which can provide money.
❧Pinkie Pie is a disaster-sensing precognitive. Having her in the bakery can possibly replace insurance. This also brings her incredible quantities of goodwill, not a thing she generally burns through quickly.
❧Pinkie Pie is a cheap babysitter.
❧Pinkie Pie knows esoteric lore, and has curbed at least one disaster (parasprites, even if she was Cassandra'd) though she's also caused some (mirror pool).
❧Pinkie Pie does pranks.
❧Pinkie PIe is reasonably-friendly with Discord. 
❧No Play makes Pinkie Pie go Crazy.
❧Pinkie PIe knows all your personal data (inc. where you live) and has a BatPartyPlanningPlace.
❧Pinkie Pie has a cannon, and has used it in defense of Equestria (Canterlot Wedding).
❧Pinkie Pie is an eldritch horror who can enforce oaths.
Pinkie Pie is awesome and wonderful.
I think Pinkie is on balance a good employee for the Cakes. Yeah, she is unpredictable, causes messes, eats up inventory and misses her shifts sometimes because a new pony entered town and needs a welcome party. But she also can and probably does use her weird powers to work and clean at super speeds when necessary.
And then there's her side business as a party planner, which ties into Sugar Cube corner in a number of beneficial ways for the Cakes. I think Pinkie's business model works like this: Everyone gets a free welcome-to-Ponyville party. Pinkie will also throw parties for free for her really close friends. For most parties though (most birthdays, cuteceneras, anniversaries, etc) she charges the host basically expenses, and she lives off the money she earns working for the Cakes.
First and foremost, ever since we saw the party cave, I'm pretty confident Pinkie doesn't conjure party supplies out of nothing with her magic, she buys them. And she's probably Sugar Cube Corner's single biggest customer, even with her employee discount. She probably pays separately to rent out Sugar Cube Corner for cuteceneras and the like. And there there's the marketing factor, every pony moving to Ponyville gets a delicious Sugar Cuber Corner treat foisted on them when they move to town at Pinkie's expense.
And of course, the Cake's don't view her so dispassionately. She moved in when she was like 12 and is like a daughter to them, one who probably babysits the twins for free frequently. If they end up short a little once in a while because Pinkie ate a gross of cupcakes, they can accept that.