Page Rank And Alexa Rank Drop TemporaryThis is the most worst point that causes many blogger to escape from custom domain. A sub domain will loose everything such as page rank,alexa rank and backlinks when it redirect to custom domain But this effect will be temporary once crawler index all of your blog pages corresponding to your custom domain you will get same page rank for your new domain in next update.
Due to new domain you can also get a sharp decrement in your blog traffic so don't frustrate, now you have to update your blog frequently to attract search engine spiders with a reason to index blog pages in relative to your new domain.
Custom Domain Attract BrandsCustom domain have greater ability to attract different types of brand and advertising option company by which you can make some impressive amount of money. Google Adsense allows sub domain to participate in their program but the sign up process is very difficult but on the other hand custom domain easily get approved by Google adsense due to its sureness.
BSA is a biggest competitor of google adsense but this service also don't except sub domain.
So conclusion is that buying a custom domain will not harm you anything it cost you only 7$ per year which is just an investment and can be easily returned when you received your first cheque by any ad network.
Increase Your ReadershipIt is well prove that number of subscriber increase to double or triple which is very interesting. Usually reader prefer to follow trusted blog that provide genuine and unique articles on regular basis. A sub domain is treated as temporary blog that can shutdown anytime because it is hosted on free service and shutting down that blog will not harm anything to author or founder. One of my friend when switch to custom domain its twitter and facebook likes just tripled. Hence custom domain will increase your subscriber rate which will further increase your blog traffic.
Finally by going through this complete article one final word I want to say that if you are interesting in long term blogging then you should go for custom domain.
Or if you have any problem or query about sub domain and custom domain the comment below to clear that instantly.