Well, I have observed some of the advantages you are offered with a diary post:

1. Diary post do not have creepy ratings. Doesn't matter how bad or how much crap do you produce with your thoughts.

2. Diary posts require low page load time. They don't posses advertisments. Wish whole MS was this way. or, They are more accessible to anyone.

3. Diary post's comments only give you a 2 point less (per comment), and give a good place for chatting and instant messaging. Which was missing on MS for all the time.

4. Diary post's comments can be edited as well as deleted, So, total remodelling is possible. Same with the posts as well. They can be deleted too!

5. Diary post is a good way of quick expression, No need of searching a proper topic or grade under which you need to certufy your writup!

6. These can be divided into selfmade folders, according to their face content.

7*. Diary's are a kewl way for screwing and accusing someone for nothing. :D

8. They have humor, thoughts, news everything a site needs to be updated every second!

*Applicable for some.
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Could only think 8 advantages. Does anybody know any more?