When not to use a subdomain

I’m a fan of the general advice of use subdomains, particularly for all those little projects we all cook up and want to put somewhere with a domain name we own and control. Subdomains instead of top-level domains, because:

• They are free. No ongoing cost that you’ll inevitably want to cut off.
• They are cool.
• You can control the DNS uniquely meaning it can point to hosting wherever
• You can always upgrade it to a top-level domain, but you can’t go the other direction.

But are there reasons you shouldn’t use one? Of course. Here are a few:

• You don’t really like your main domain or it’s not appropriate somehow. A subdomain will lock you further to it.
• The project is very similar to what your main site is about. You might consider a subdirectory then, because you might as well get the SEO juice from same-domain content.
• You need to share sessions. If your main site has a login system and that matters to the other project, a subdomain is (probably) out.