If you never blog before, Google blog is a good place to start practicing the craft before getting a permanent blog for yourself. Or, if you prefer, use Google blog as your permanent place to leave your blogging work as a portfolio for yourself and potentially, for future work as a blogger for other people. In this day and age, article blogs are the place to refer to when potential client is interested in hiring bloggers for their expertise in different niches. However, thanks to Google blog, you have the opportunity to show what you know due to your blogging experiences.
Since Google blog is free, you can decide whether to take advantage of having your own domain or use the sub domain. From a professional point of view, it is better to purchase the domain just so people can easily find you online. I am not saying that nobody can find you with the sub domain but if you want others to take you more seriously, just paying a bit more money will make you stand out. It does not cost a lot of money to purchase the domain especially if you buy it from a reputable and a well known online company that have a good reputation. Some good ones that come to mind are goDaddy and BlueHost. The difference between the two is that one is a one time payment while the other is a monthly fee. Your decision whether to pay a one time fee or a monthly fee for a domain should take into consideration especially if you are the one paying for it.
I do not know many bloggers that use the Google blog format on a professional level. But it can be a valuable resource to use if you have no other option to talk about what your interests are. What matters most is that you got started with blogging instead of waiting and dreaming about it. While you are at it, take the time to learn a thing or two of online marketing. What you can get out of it is free traffic and potentially new clients that would be interested in using your writing and blogging services. Instead of you looking for your clients, let them find you which is due to the keywords. You never know who will hire you as long as you do not take action.