How much time do you have?
No, really. I mean on the shoot. I guess it depends on who's running things, and the particulars of each shoot, but my experience is actually the opposite of Ernest's -- using the camera's display is the quickest.
I will agree, though, that you end up with a higher percentage of shots that just aren't quite right, when you use the camera's display. When you finally see it blown up on you computer's monitor, you'd be surprised how many things sneak in that you didn't notice when watching on the puny camera's display.
So, to that end, I think the benefit of a monitor or Z-finder is that it allows you to more easily catch those little details that are actually quite huge. And, as you notice all those little differences, you're going to spend more time fixing them, on set. Which is why, in my experience, a Z-finder actually adds time to your shoot, cuz you're gonna notice more stuff. That's good.
So, do you have time for that? Do you have time to pay attention to every little detail? Kudos to you, if you truly do. To be upfront, this is an area in which many (maybe most) people on this forum disagree with me. If I can paraphrase their logic, they say that you should take your time and do it right. By my logic, at the extremely-low-budget level of filmmaking, doing it right means doing it quickly, for many reasons.
So, how essential is a Z-finder? I dunno. Depends on your style of production. Me? I'm a run-and-gun kinda guy; I don't have time for a Z-finder. Are you like me? Or, are your shoots a little more deliberate? If so, you might really like a Z-finder.