Is there any advantage of taking backlinks from web 2.0 Niche sites

how does niche sites will help in site ranking

Well i think it actually depends upon the competition of your niche. Sure though, it can increase or add (some authority) backlinks to your site. But just like I said it depends on your competition.

Depending on the niche it could be easier for some to be an authority - still, I wouldn't take that as google finding a "Dog Nail Clipping" blog to be credible because there aren't many blogs like it as it would be a micro niche. Unfortunately, you need to stick to mainstream tricks here - the advantages in being a niche though are that you should rank higher on keywords relevant to your niche than a site with similar traffic and backlinks with a less specific focus. The advantages in a micro niche are that often, there is less competition.

only normal web 2.0 exists .Niche based web 2.0 is not available .but getting niche based links from other sources is good for the link

web 2.0 is ok now, but you should to make diversity keywords in each post, it should be better for your niche