The hippopotamus site that is linked to from many zoo pages is more likely to increase its ranking than the hippopotamus site that stands alone and is only linked to from one site. The theory is that if the site was important enough to warrant a review, it will be of more value to the user than a site that hasn't been reviewed. As for meta tags, it's a gamble. Some engines heed them, others ignore them. It is definitely possible for a site with no meta tags to achieve a high ranking on a site.
Many search engines are sensitive to spamming.
Sometimes, good intentions can create bad side effects. Since a bot's main function is to download pages, it can be demanding on a web server. There are several key elements of good robot etiquette. Suppose you have sensitive information on your site that you don't want an indexing bot to advertise. Or perhaps you have temporary or developmental html files that are not intended for the public domain.