"Why can't the usernames be like email addresses?" [in the sense of querying DNS to find them].
In this context we're talking about what it takes to avoid relying on DNS (because DNS is a centralized, highly politicized system). Your solution would still rely on DNS.
A system which relies on DNS, but in which anyone can setup a node seems better than a system that relies on DNS but only has one centralized node (Twitter), no?
In a truly decentralized system, you're not going to be able to have readable unique names without collisions. Why? What happens when the network splits, then people on either side of the split setup the same username. How do you rectify this when the network rejoins? How do you know that the network has split vs. a node going offline (if you wanted to do something like shut down new usernames until the network was whole again)?
"A system which relies on DNS, but in which anyone can setup a node seems better than a system that relies on DNS but only has one centralized node (Twitter), no?"
The concern here is that whoever is currently leasing the domain name has authority over users' identities. A better system would let users own their identities outright.
"In a truly decentralized system, you're not going to be able to have readable unique names without collisions. Why? [....]"
This is a well explored topic. A good place to start might be to look up "Zooko's Triangle" and then go forward from there towards various ways people have figured out to deal with such problems. (Zooko's wasn't the last word.)
[1] http://en.wikipedia.org/wiki/SRV_record