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Is this true?
I think it IS a good idea actually, as it would cost a bomb to continually purchases fresh domain space for each new keyword site you build around.
1) Do the subdomains make sense? In other words, if you have one site on puppies, another on the life of Mother Teresa, and a third on municipal bonds, does it make sense to have them all under the domain "mysite.com"?
2) If mysite.com gets whacked in the search engines, won't the subdomains go down with it?
Finally, if your plan is to create a whole slew of sites or subdomains on a zillion topics that you know little or nothing about (as too many AdSense publishers do), are you prepared for the day when that strategy will no longer work?
When I started our main site I did use sub domain for the big categories like movies, world news, arts, tech etc. I thought it was a handy way to divide things up and even for users.
However, I did not realise some of the complication regards search engines looking at them as different sites and making stats hard to analyse sometimes.
Also it can encourage over-reach. We moved out of entertainment into all sorts of hard news and now two years later we have decided to slit that hard news off to a separate site.
Don't put all your eggs in one basket and read up on sub domains before you go down that route.
When you create subdomain it is crucial to make sure that your traffic also comes from sources other than search engines.