I don't think you want to be living a life where you have to issue ultimatums to get what you need. The world is a bigger place. Find better work and go and do it. You can remind them of the things you've been unhappy about by leaving. If everyone did that (instead of sticking in dead-end jobs) it would be much harder to create crappy jobs.
Yet companies act in exactly that way towards their employees. Late too many times? Ultimatum is issued where you get fired if it happens again. Too many sick days? Same result. Underperforming? Yup, you guessed it. Your employers are not your friends - do what you have to to make your life easier. IMO, ofcourse.
A good employer will talk to you and try and work things out before giving you an ultimatum. You lose little, and can make things a lot easier, by doing the same.
For most people quitting your job is an extremely risky proposition. Even if you do have a financial safety net, there are no guarantees other employers will be jumping at the opportunity to give you a job.
