Deleting Blog comments is a very personal decision. It is always up to the Author of the Blog to decide if something that is left, is of value to his audience.
Some seemingly don't care who comments and if those commenting are adding a 'selfish' plug about their business, that might not be anywhere near on topic. In the long run this does not help the Blog Operator - it hurts their indexing.
Here is a super short video I put together showing examples of some of the comments we get on our Main Real Estate Blog platform. With my Reasoning as to why I would keep or delete them.
After you watch the video, comment below. Do you agree that not all comments should be accepted? What about the unrelated ones, do they irritate you as well?