Long sales pages give you more opportunity to make mistakes... so you could have a great headline but maybe your logical argument is keeping people from buying.
But a short squeeze page will keep most of the attention on the headline and make it easier to find a winner.
Squeeze pages also can perform better with cheap penny traffic than long sales pages, so you can add a few headlines to your multivariate testing software and send a few thousand visitors to the page for $10-$30 and get a pretty accurate result. (I've used Clixsense for this in the past.)
This is also just a good way to practice your copywriting skills and see how they match up against real visitors.
I wrote a blog post on writing quick effective squeeze pages here: 7 Steps To A Successful Squeeze Page