If you don't put robots.txt file then there is no any single disadvantage of that. But SEO point of view if you put that file then it is beneficial for you.
No any disadvantage but robots.txt file is used for restricting any directory of file which you don't want to crawl by the search engines.
Agree with Alan Smith. No issues if you doesn't insert your robots.txt in your site. It, only benefits you if you include it in your site as SEO point of view. Wish to share an article from SEO Complete Solution which provides the benefits of robots.txt & explains it very well. Go through it.
Websites that have undergone Search Engine Optimization may include robots exclusion standards, or robots.txt, which are text file protocols that control the search engine spiders. They advise the spiders to refrain from visiting parts of the Website that are either irrelevant to the site’s categorization or that may be private.
That’s a great question. One of my blog I’ve a static robots.txt, so I’ve set up a dynamic example using code snippets from other sites: this robots.txt is what a user sees, and here is what various crawlers get on request of my robots.txt example. Of course crawlers don’t request a robots.txt file with a query string identifying themselves. Imdad
It will help spiders to crawl all your pages. If you want, you can block the spiders through robots.txt
If you dont want search engines to crawl any web page, then you can use robot.txt on that page code. There is no disadvantage from this code.