When I first started this some years ago. I use to think that PR or pagerank was the do all and end all of SEO. The first thing I would look at when deciding to link to a website was PR. After years of trial and error, I have personally experience just how much and how little PR really means.
Firstly, PR was named after some guy in google who created it. Larry Page. He decided to give websites a rank based on how many website are linking to it. Think of a incoming link to your website as a vote for your website. But again, its not that simple. These "votes" need to come from relevant and trusted sources.
If a gardening website linked to a sports supply shop - even if both websites already have high position in the SERPs. It would not be counted. You have to be smart with your links. I read on a blog some time back about job openings for Link Ninjas. I think that is a cool way of calling the job. Think of all the ohhs and ahhs when introducing yourself in a bar.
Girl: "So, what do you do?"
Boy: "I'm a link Ninja"
Girl smiles shyly.
Boy, modestly, "Well, yeah I have a samurai sword in my backpack next to my laptop."
Links essentially make up your page rank. But, you don't have to have a high page rank to have a high position in the SERPs.
SERPs = Search Engine Results Page
I am managing an accommodations website with a PR of 3 and its on the first page of around 6 major keywords. It also has the correct semantic structure and complies with almost every SEO parameter. Its a great site but its PR is only 3/10. Would I still pursue a link from it? A resounding YES from a Link Ninja!
PR is just one of the factors to consider when linking. Other factors include counting how many external links are leaving the page, making sure that its has content relating to yours (title, headers, text), considering where your link is actually going to come from, etc.
If your website is showing up in the first page of your major keyword, would it still be worth it to pursue links to increase your page rank?
Yes. Always. You need PR because Google uses it to determine the importance of your website in the food chain. And if Google uses it, you HAVE to use it. Also as more and more websites are getting optimized, its always good to stay ahead of the pack.