So in effect you are either suggesting to:
find a niche without an authority and build an authority although you could do just fine with MNS (because there is no authority)
OR
find a niche with an authority and start building a site to topple that authority?
Please don't involve the domain costs because if someone is not earning at least a $100 per domain / year he might as well drop that domain.
You're still comparing mns's with authority sites. An authority site targets 1000's of keywords minimum. An authority site should be making upwards of $10k/mo.
A mns targets 1 main keyword with a few smaller keywords it'll get too. If you can make $10k/mo with 1 micro niche site then you must be one rich man and I take my hat off to you.
To answer your question above you would find a niche without an authority site. Let's take warcraft for example. Pretend it has no authority sites.
A mns targets "wow priest guide" and maybe "wow priest build" as a secondary decent keyword. It'll get hits for a few very longtail keywords too.
An authority site would target, all the "wow class guide", "wow class build", all the professions like "wow engineering guide", "wow engineering build". So far that's about 40-50 1000 word articles. Next you'd have talent builds. Raiding guides, items guides, news and opinion about upcoming changes, maybe a forum on there for discussion.
It's a completely different endeavor and essentially a full-time online business. But if you pull it off you'll have traffic coming in from all over the place from wow keywords and when you added new content you would not only rank well quicker, but you'd get your existing traffic to that page from other existing pages.
From there you can have people +1 it, like it, share it on various social media, bookmark it etc. The whole thing starts to expand exponentially if you crack it and suddenly you have a thriving community making you a small fortune.
You can also choose to go after niches that have say 1 existing authority site, or if you're really in it for the long term you'll go after ones with strong or multiple authority sites. Or if you just believe you can topple them with your business plan, and it does come down more to the business plan for an authority site rather than the SEO because people in that niche will start to see your new authority site and if it's not as good as the existing ones they won't bother using your site.
I mention the domain costs because for a newbie they can't just go and splash out $500 on 50 domains and it can take time to get paid. Most people that try mns's also fail and end up making the odd dollar here and there or just don't rank. It's not as easy as just registering an emd and adding a couple of backlinks otherwise we'd all be making at least $2k/mo online with ease.
And of course the main issue with multiple mns in the same niche is you need separate IPs so that's not only additional cost, but hassle to get, especially if you want them on separate class c's. Then what if you want to put analytics on them or use google webmaster. You can't exactly just put them all on the 1 account otherwise you've just told google you're trying to run a little niche farm.

It's just a major hassle to do mns's on a large scale.