So, if I have a lot of comments, will this help me rank in SE. I get a lot spam comments on my blog and I trash most of them. But maybe I could leave them? So, should I? And can they slow me down if they link to some type of sites, like gamble site or something?
Depends on what kind of blog comments. I have seen almost plenty of spammed comments dropping their anchor texts and links.
You want to ensure your comments do not take away from the relevance of your page. So if a comment is left that has absolutely nothing to do with the blog posting itself do not publish it. Publishing these comments will result in the page loosing it's keyword relevance due to all the non sense posted in the comment section of the page. Keyword density is diminished very quickly. Not to mention your visitors will cease to visit if all they see in the comments is spam. Always approve comments relevant to the blog post and trash the rest.
Bolg comments are a part of seo if many seo professionals are following it to get placed into SERP. Now there are few norms that should be followed to get the effective result from blog commenting. 1) Choose relevant Blog 2) Try to comment on blogs that allows dofollow 3) use keyword in the field name 4) check that whether the blog has approved the comments within last 6 months (as their are many blogs that are not actively involved in comments approval) 5) Try to maintain the keyword or the theme of the blog in you comments. That can make your approval easy.
You should keep the comments that counts worth or that can help you to rank higher for some more keywords. If you don't want to give any link to any anyone then you can just remove the link before comment approval or you can make the links nofollow.