Why spamming? -- Spammer automatically post some random, useless comments not even reading the content, add links that points to the product they have promoted or blog sites to basically increase the result of search engine ranking, push the site to the top ahead of the competitors site to increase the number of potential visitors and convert it into money (sales).
The Worse Comment i have ever read: "read my blog - link"Isn't it annoying? i think so...because of this abusive acts of spammers most of the bloggers or webmasters tried a lot of possible solutions to minimize the comments spam and if successful trash the comment to the bin. Some of the solutions uses are nofollow, blocking by keywords, dis-allowing links in the comment post, anti-spamming software and by switching off comments -- their so much methods or solutions bloggers have tried to make everything clean and fair.
So therefore if you want to leave some comment make sure you read the post content, understand it and only comment if their is something you want to add or say that is the main purpose of commenting but, if your reason is just to spam and wanted to be trash then go ahead because you're going nowhere but in the bin, you won't get the link...so why not doing it the right way.