k i told my friend this doesnt work good but i couldnt tell him exactly why to make money with adsense he wants to know why buy so many different domain names instead of making one site and it work it something like this site.com/surfing site.com/basketball site.com/sat-test
The only problem I see with this is that when you go to submit to directories, you will only be able to give them site.com.
Technically for SE rankings subdomains would work the same as top level domains, however practically subdomains are much harder to market and promote, for example for the reason sayyes mentioned.