Since most of the blog comment links come with no follow tags they do not offer any link juice. But if you wish to improve your ranking in Yahoo and Bing which do not understand no follow tag, you can try it but make sure that you are posting something really informative and not anything just for the sake of getting some links.
Comment spamming is shit!
I could not get what you want to mean here. Do you want to mean that it is search engines that decides which links to follow and which not? Please explain.
It's less and less valuable to comment on blogs. Also google is penalizing sites that use massive blog commenting.
Is blog commenting good for SEO? Yes and No. The value of a link is very low. When it's from a non related website is useless. But as one of the things you can do for SEO is a way to build diverse backlinks.