A domain name should reflect the site which it represents. How is anyone suppose to know what the domain a.com or b.com would represent?
In my opinion, when you buy a domain you want to attract interest to the site or product. How in the world is a.com or b.com going to do that?
Not to mention, there will only be 26 of them, so buying one will be nearly impossible, as well it will be more or less of a popularity contest to say you own the domain a.com, or z.com.