suddently think of this today, by using the meta index nofollow, this can prevent the bot from crawling to external site, and thus preventing links to unrelevant site, and to bad neighbourhood as well. is there any proof that this helps page ranking?

If I remember correctly, it's the sites linking to you that help your ranking, not which sites you link to. That's why spammers try posting to forums and blogs; they're trying to capture links from a well-known site. It shouldn't matter who your site is linking to. Hence, using it or not won't make a difference, except to those sites getting traffic from you.

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I fink nofollow you never place on your site if you dont whant that the spider index the links dont but them on your site it is that easy

for example you have to link to site but you dont want google to follow this link then use nonfollow attribute.
In a sense it will help your PR, because your page rank is also spread to the sites you are linking so by stopping google from following the link that page PR is going to stay with that page and you are not giving any to the site.

That is why Mattew Cutts blog which is a PR 6 is using this non follow, actually most wordpress blogs use this for comments so I dont underestand why some people continue to spam blogs.
I have like 5000 comments to delete on my blog.....

"nofollow" is a great tool for a number of things, and it depends on what you are wanting to use it for.

It should not, however, be confused with Meta Elements or robots.txt, which should be used if you are trying to block access to certain content or keep certain content out of search engines.

You should research which search engines you wish to have which effect on, because they all treat "nofollow" differently.

Some of them (such as Ask.com) don't pay any attention to "nofollow", last I checked.

Google and MSN.com have had mixed reviews (what they claim they treat "nofollow" as versus how "nofollow" actually functions with their search engines).

Overall, the purpose of "nofollow" is not to cut out potential userbase, instead, it is to make sure the ratio of users referred to your site by a search engine are quality users that are trying to learn more about your product.

If "nofollow" is properly used, you can cut out a lot of spamdexing. And "spamdexing" is a horrid thing to have plague your site, as it is sure to reduce the conversion rate of your impressioned users to solid sales as well as reduce the effectiveness of any advertising revenue (and thus the amount of such over time).

Wikipedia has great entries, and I highly recommend you to research further to make the most out of your site.

meta nofollow will help to increase your PR as mentioned above but not sure of that.