Multiple sites are appealing to me. It's just a shame that Google only allow upto 50 active channels to fully monitor each. 100 would be a great challenge.
having multiple niches on the same site wil definately cut ur eCPM! that is whart google call smart pricing!
I think 2 site is good. 1 site is risk, more site you don't manager good content. 2 site is best choice.
I would go with 1 or may be 2 or 3 sites and not more than that at a time. Update each one every few days and I guess they should be fine !!
Mahalo.com does something like that and I think that site is earning pretty well (it also have the Premium Google Ads...). On the other hand, it's pretty much impossible to build a site like that without the help of other people.
I think it is great to have more sites but you should have 1 site that you can put all your other sites on. it is good to build sites with simular related content. if you can group all your sites together then you can drive traffic to that 1 site and get traffic coming to all your site because they are related.
While agree with the original post, and see it as sound and constructive advice, my problem is I have a bunch of interests and a site to represent each. It is proving to be hard work so I am looking at combining several sites/blogs and have 2-3 top notch sites that I focus on. I guess in the end, just do what works for you.
Multiple niche on the same site will increate its sale, visitors for different neet will imporve its performance and traffic.