Many advertise services are do not accept Subdomains
Pagerank Increase slowly with subdomains
The Google is one of the best page rank giving for websites. All are known that Blogger is the one of the part of Google. Many times you writing many articles and you can't get any traffic due to using of your subdomain. Friends if you visit a domain website you will find all blog links are added with "no follow tag" (you can see this a website by clicking view selection source) because if your website a day delete or broken then her domain website will get decrease her pagerank.
Fan and subscribe list Increase in Domains
The Blogger is gives to all to start website then change the website to domain if you are interested to earn revenue. All visitors are come to your website and see your website with subdomain then they will get bad impression for your website then they can't subscribe or like your website any post.
You should buy only a domain not for hosting
The Blogger is one of the best Hosting service. you don't need to purchase a hosting because blogger is giving you free hosting service and you only need to purchase a domain. Many peoples are not like WordPress due to purchase of hosting service. I thinks o it is one of the biggest reason for peoples are choosing blogger.
Ranking will speed up
After you purchase a domain you will get more no of visitors, subscribers, fans and also your visitors are share your posts to their social page. Your social page is giving you more visitors then your Alexa rank will speed up and you change your traffic to money. I think so .com domains are giving you highest traffic because these domains are high popularity in web world.
Page rank will grow up
Your blog will mot getting any page rank to writing much more articles because blogger is a subdomain. After using a domain it will grow your page rank. If your blogger is so much older, it will already getting pagerank then don't worry because after purchasing a domain Google will reboot your all blog links and you will get your page rank.
You can increase your business
After purchasing a domain you can start and offer to your visitors to start guest posting. You can give permission to sponsors, advertisers to get advertise in your website. Advertisers and sponsors they advertise where they get huge amount of traffic.