Is putting the www prefix necessary for a domain URL? Does it make a difference in making your site more legitimate? Does Google prefer having it?
No difference what so ever, you should only use one though. If you can access your website using WWW and without WWW, then that's going to hurt your SERPs and sites SEO and you should change that ASAP.
It doesn't matter may friend..just type you site and .com and keep it moving...the search engine will still find you.
You must decide want to use www or not. because search engine looks with www and without as 2 page. So, if in the beginning u use www, always use www
Pick which one you want and do a 301 redirect from the one you don't want to the one you do want. Google isn't fond of you trying to rank http://mysite.com/ and http://www.mysite.com/
I'm thinking more along the lines of: does the general public see a site with www as more legit or do most people not even look at the URL and just care about content?
Generally a site with the "www" prefix looks more appealing to the generaly public...but in regards to SEO...it really does not matter, as mentioned above, all you gota do is make sure that you choose one in the beginning and stick to it. Google will see the two versions as two different sites, and only one will be ranked...so concentrate all your backlinks on only the one you choose.