I think to understand what is what here first we should divide niche blogging into two categories based on what particular blog offer to visitors: 1.Opinions and thoughts on some Subject(s). 2. focusing on Products like cameras, cars, software etc. Here I assume that either subjects not invented or products not produced by blogger. (It is evident that problem of original content doesn't arise if blogger is himself producing products he talking about) Main assumption I make here is that you want to attract audience.

Now having these two major blog sphere categories, let's apply our question of Original Content. Now, when we made such division, some evident things appear on a surface.

Syndication approach in filling content on a site which is focused on some problem or discussing some subject - is dead approach in terms of attracting big audience. There are few cases when I returned to a site from which I found a link to a interesting article, unless there are a lot of original and interesting material.

Since the blogger did not invent the subject but just focusing on it, I expect him to cover subject with his original thoughts and material. If the blog is just a container of excerpts of other authors articles with links then this blog looks like incomplete collection of links on subject. And I better google for subject next time.

On the other hand, when the subject is a product (cameras, phones, gadgets etc.) then syndication approach may work to some extent. You call your site “Bla-Bla-review blog”, you put some information on a product (I don't think that product manufacturer will sue blogger for reprinting specifications of a product?) and make short excerpt with a link to product review article (that is what some professional bloggers do). Add search engines love of blogs, some time (do not forget to post regularly new products information), mix content with ads, apply some SEO techniques and you may at least cover your hosting expenses.

With this kind of blog you get a lot of visitors going to buy a product and searching the net for reviews and opinions on this product. You will not get a lot of coming back visitors, but a lot of visits through search engines.

Sure, I simplified the question and there are blogs focused on interests and alike and many just don't care. But basically, when your primary goal is to attract visitors then you should consider this issue. These were just my thoughts.