SERPs are influenced by A LOT more important factors than PR...
Absolutely. But in this thread I'm only addressing the specific factor of PR. It's definitely not the most important factor, but it's one that's quite often misunderstood.
The problem is that most often people explain everything necessary for SEO in one thread. That's fine when discussing SEO in general, but when explaining individual factors, it's best to stick to just that single topic. Otherwise new people get confused about what each individual thing is. (Since the topic of this thread is PR, I guarantee there will be at least a few people that read this thread who will think all the factors you included in your last reply pertain to PR. lol)
Actually that post you made would make a good thread on it's own regarding the "big picture" of what's important for SEO in general.
So while all those things are very important for SEO (some of them are much more important than PR), this thread isn't about all the factors necessary for SEO. This thread is specifically about when, why, and how PR affects a pages SERP position, so it's worth mentioning for those individuals who might be confused that the factors you listed have nothing to do with PR.
the only reason left to care about PR is if you want to sell links to costumers also the PR does not determinate the quality of a particular backlink
Well that depends on what particular thing you're looking for.
If you're looking for relevant blogs, then PR doesn't mean quality, relevance does. If you're looking for domain trust then PR doesn't mean quality, trust factors do. But, if your looking for quantity value of backlinks then PR is a desirable quality.
Actually a very high quality backlink is one with all factors combined. ie. a targeted keyword anchor, on a relevant page, on a trusted domain, on a higher PR page, and when possible within the main content of the page.
A backlink on one high PR page can be the equivalent of hundreds or thousands of backlinks on low PR pages. If that page is a relevant page then that one backlink will be like having hundreds of backlinks on relevant pages. If the page is unrelated to your niche, then the backlink will be like having backlinks on hundreds of unrelated pages.
Which brings us back to the main point in the thread. The value of PR (as far as it's value for the SERPs) is not about having a high PR page of your own, but rather in finding high PR pages to get keyword anchored backlinks from.
As madoctopus explained in his reply above there are many other factors for SEO besides PR that you'll want to look for as well. The point of this thread though, is simply to clear up some misunderstandings a lot of people have about what PR is and does. It's definitely important for people to remember that PR is just a single factor among many necessary for SEO, but it's also good to actually understand each single factor on it's own.