Is it recommended to use both tracker and google analytics?
What do you guys use google analytics to view? I know it can see how visitors react on your site, eg bounce rate, time spent, eg.
Are there any other useful stats on GA that are useful for us, as affiliates?
It's usually not necessary, but you can if you like. Honestly I wouldn't usually bother: most GA features aren't much use for classic one-pager affiliate campaigns. (Although if you would like to track Javascript events, then GA becomes very useful - I wrote a tutorial about this a while ago.)
Time spent stats from GA are highly unreliable for single-page sites, so I wouldn't trust them. If you want to track time spent, custom Javascript is a better way to do it. This is one decent library for the purpose: https://github.com/jasonzissman/TimeMe.js?files=1