Locating blogs in your own niche as well as others of interest to you and commenting on them is a logical step to take when you begin blogging. The two are NOT mutually exclusive choices. I know know any bloggers who read and comment ONLY on blogs in the same niche their blogs are in. We bloggers like all humans have many interests and we read and comment in blogs in a variety of niches.
If you are simply blogging as a form of socializing and making friends then backlinks and pagerank will not be of interest to you. If you are a business blogger, a professional blogger, a charitable organization blogger, or almost any other blogger, who wants their blog to be considered to be successful in any niche, then reading and commenting on blogs in your own niche is a natural thing to do, as it leads to forming relationships and links are an indication of relationships.
1. When someone is new to blogging it makes sense for them to locate other blogs in the same niche and to comment on and backlink to those blogs in posts of their own on the same subjects. .
For example you are writing about "elephants" then you will probably be attracted to other blogs where "elephants" are being discussed. You will want to be informed about the latest news pertaining to "elephants" so you will research the topic before you publish your posts on it and include links to relevant articles in them.
The reason we provide links to other related articles in our posts are 4 in number. Links to your sources are important to your readers at least four reasons: Verifiability, Acknowledgment, Examples, Context. If you don't even read blogs on the topic(s) you blog about then readers may wonder where do you get your information from, and how much credence they can attribute to what you publish.
2. Commenting on blogs in your own niche brings the fact your blog exists to the attention of like-minded others and their readers in the same niche who blog on the same or similar topics.
3. Commenting on blogs in your own niche may lead to the bloggers who receive comments from you to click your username link, click in and read, and comment on your posts.
4. Commenting on blogs in your own niche may lead to the bloggers who receive comments from you backlinking to your posts in blog posts of their own on the same topic (s) (And do note that without backlinks to your posts in posts published on the blogs of like-minded bloggers your blog will not earn a pagerank.)
5. Commenting on blogs in your own niche also brings your new blog some traffic from those readers on related blogs you commented on, who click your username link in your comments and visit your blog to see what you have to offer.
6. Commenting on blogs in your own niche may lead to readers of blogs who receive comments from you choosing to backlink to your posts in blog posts of their own on the same topic (s)
It's my experience it is only those who have no specific purpose for their blog or goals they aim to achieve via blogging, who raise objections to being told to locate and comment on blogs in the same niche. They blog for the purpose of connecting and socializing and and want to make friends and have fun. They don't care about niches, links, backlinks or pagerank.
Well, the most wonderful thing about the blogosphere is that there room for all kinds of bloggers in it, including those who actually want to learn how to blog effectively and achieve their blogging goals. So blog on!