Even if you are running a self hosted blog by using WordPress.org blog software, you definitely need a WordPress.com account. There are many advantages of a WordPress.com account as given below:
1) Visitors from WordPress.com: WordPress.com is a full-fledged blog social network and you can connect your self hosted WordPress blog to a WordPress.com account. From the WordPress.com account you can follow other like-minded bloggers and they could follow you back. From the day I have connected my self hosted WordPress blog to my WordPress.com account, I have seen visitors coming from WordPress.com to this blog. After you connect both the accounts, post new content and after few days in Google Analytics, go to: Traffic Sources –> Social –> Overview. Here, you can see visitors arriving from WordPress.com. If you are posting high quality content in your blog, then you might get featured in “Freshly Pressed” section. If that happens then you can anticipate a huge spike in traffic.
2) Jetpack: I love Jetpack plugin and if you want to use it, you MUST have a WordPress.com account. Jetpack has plethora of other advantages.
3) Gravatar: This is another great advantage of a WordPress.com account. Gravatar is a small profile picture you see next to your name when you posts comments. To create a gravatar you MUST have a WordPress.com account. Another advantage of Gravatar is that you get a free profile page at Gravatar.com where you can add your Twitter, Facebook, Google+, LinkedIn and other accounts.
4) Akismet: Akismet is one of the best plugins for stopping comment spam and you need a WordPress.com account for generating an Akismet key for using it in your blog. Personally, I don’t use Akismet, I just use the Jetpack comment system and it has helped me in stopping spam comments.
5) Polldaddy: Polldaddy is a classy survey and poll tool for WordPress and you need a WordPress.com account to use it in your blogs.
6) Intense Debate Comments: If your blog receives many comments then you might try Intense debate comments. Again, you need a WordPress.com account for using Intense Debate. Personally my vote goes to Jetpack comments.
7) VaultPress: If you are concerned about the security and backup of your blog then you can try VaultPress. You need a WordPress.com account for using ValutPress. Personally, I use automatic backups which is provided by Hostgator.