I believe Bing doesn’t like duplicate content, so has banned one of my sites; because my site is ALMOST ENTIRELY DUPLICATED CONTENT. But that misses the point.
There are many reason why you’d want to use Site A rather than Site B to get the same information.
Some obvious ones:
- Site A is faster.
- Site A doesn’t have ads
- Site A has an index
- Site A is cross referenced
- Site A’s 0.001% original content
In fact, the 0.001% original content could be the only reason needed for a site to rank well.