Suppose you have a brand new website with a WordPress blog. As a serious internet marketer/business person, you would want to make full use of the blog for the uses of interactivity and marketability.
You understand the difference between using a DoFollow or a NoFollow tag, but are not sure which ones to use for your blog. The choice really depends on what your personal preference is.
As you might already know, DoFollow tags pass link juice to their target. For example, if a person comments on your DoFollow blog and leaves a link to his website, he will get link benefit from his posting. A NoFollow link is just that: It does not provide any link juice to its destination.
Now a natural question arises: Why would you want to provide your visitors with valuable link juice?
Ideally you would not. But the catch is, Google (and maybe other search engines) look at you more favourably if you have outgoing links from your website. In this case, having people leaving links on your blog can actually help you in becoming an authority website and dominating your niche.
So in the end it really comes down to your personal preference. If you keep your blog as a DoFollow blog, you will get loads of spam, because people will less brainpower and automated scripted will look forward to stealing some of that link juice. But a DoFollow will also provide users with more incentive to comment on your posts!
If you choose to have a NoFollow blog, you will still get a lot of spam (simply because a lot of spammers do not really know that their postings on NoFollow blogs are wasting their time) and will mostly reduce the interactivity on your blog, because most people will comment on your posts. But people who are looking to provide meaningful feedback and who do not care about the link juice will keep on commenting.
I would personally have a DoFollow blog, because outgoing and incoming links are both very important factors to determine your search engine rankings. Remember, if you give out to others over the web, most people will feel obligated and do you favors. For this reason, if you let legitimate blog comments contain an outbound link, other websites will also link back to you.