Question about sub-domains

[admin]

Hi,

Here is my situation :

I have about 3 or 4 web sites on non TLD, (read: free hosting w/no ads).

In the future I plan to add at least 3 or 4 more sites.

My idea is to get a domain , something short like xtob.com or whatever then host all of my sites through subdomains such as :

coolstuff.xtob.com

Are there any reasons this may be a bad idea ?

Thanks in advance,

-me