Your friends are right about WP plugins.
Personally I never used more then 5 plugins.
Is there some rules? I don't follow rules, use plugins that you need for your WP.
There are various purpose of plugins, depend what you want with your website you will use that plugins.
Some of themes have built-in plugins like social share, contact page (contact plugin) etc. and you don't need some of plugins.
PROS can help you to speed up webpage, customize look, connect some services (mail subscription, sharing etc.), secure WP etc.
CONS too many plugins make high load to many http request to other pages, too many scripts etc. , some of plugins is hard to integrate with themes and change style.
It is best to use just plugins you need