To use or not to use....thats the question
 

I have been building sites and blogs for quite some time now. I have never used the xml sitemap plugin. Is this something that is a must have? I'd like to use it if it will be beneficial. Is it worth it or not?

Every site should have a proper sitemap.

yes its a must have.

use google xml sitemaps plugin

and in robots.txt use

Sitemap: http://www.domain.com/sitemap.xml

this way yor sitemap gets noticed by bots.