Using 'nofollow' = Bad SEO Practice?

OK, the whole concept of 'nofollow' is still a little unclear to me.

Right now, I have set this to all URLs, but I read somewhere, that this might have a bad influence on my SEO.

I thought, adding 'nofollow' to all URLs would prevent spammers from posting valuable links without permission. Placing valid valuable links on the other is realized using LinkBacks, right?

That's how I understood the whole thing.

So, if I'm using LinkBacks to have control over what links are placed and what links are not, is it still bad practice to have other links "crippled" by 'nofollow'?

EXAMPLE:

I'm posting daily news, in which references to companies or other sites are contained in the form of a link.

If the site I'm referring to is trackback-enabled, I use LinkBacks for placing a "real link", which will result in the other site backlinking to mine.

So, why should I give away links for free to sites, that do not backlink to me? Wouldn't this be irrelevant for the SEO of my own site anyway?

I mean, isn't the use of 'nofollow' a means to avoid undesirable linkbuilding?

Thanks!

Sadly, that theory has been ridiculed by Wikipedia. They put nofollow tags to all external links but hey they are even stronger than ever.

Well, for me that's not sad news at all! If this is true, I'm glad I don't have to change a thing!