It's really quite simple: you assume it's all worthless, because that's how it starts and that's how it generally ends. Even if it were going to be worth something, you'd have been better off taking the money up front and investing it however you wanted in the meantime. But it doesn't really matter, because you have no control over this anyway. All you have to do is decide whether the salary is high enough; the rest…
As an employee you can also have a positive effect on the outcome of the company.
No way. I have been screwed often enough by companies that I am no longer going to put in uncompensated additional effort more than once per employer. If I want to do charitable work, there is no shortage of open-source projects out there that would pay in gratitude or reputation.
When I am "at will" in a company with zero actual ownership, I have no interest whatsoever in seeing the company succeed or fail. It makes no difference to me whether the job disappears because the company goes under, or because the CEO doesn't like my face, or because the owners sell out and the new bosses clean house. Any non-cash incentives will have no effect on me until they are actually in my possession. A promise that can be broken without consequence at any moment before the expected delivery date is no promise at all.
As an employee-owner, I can have a positive effect on the outcome of the company. But not once has any of my employers fully upheld any promise related to ownership made at or before hiring. So now all my accounting is by what I have in hand. If what I have is a load of tissue-thin promises that I have no means to enforce, that's great for wiping my backside with. I believe it was put more delicately some time ago as "don't count your chickens before they're hatched," and "a bird in the hand is worth two in the bush."
If you can't meaningfully enforce the promise, it is worth nothing. If you have a vesting schedule, and the employer can change it by firing you without cause, it is worth nothing. Analyze your proposed compensation defensively; pretend that the company is out to screw you without you even noticing it, because sometimes they are.