Having your own domain makes you look more professional. While it may not make you a pro-blogger, he ad networks will take you a lot more seriously. Some ad networks will not accept a site unless it has its own domain name. If you do it right, having your own domain and web hosing won’t cost you anything because the income the blog generates will more than offset the cost.
It’s better to get the domain name at the start than down the road. The last thing you want to do is build up a blog with Blogspot, get a ton of backlinks, PageRank, Alexa and Technorati rankings, and then have to move it to its own domain and start over.