When to Use Separate Site Collections

I had some questions today from a respected colleague about when to use separate Site Collections, and I sorta liked my response, so I’m going to foist it off on you, too.

The gist of the questions was this:

They are fighting the battle of site collections here (IT wants several) which of course will make it impossible to share lists, site columns, content types.  Will separate site collections also effect “send to” and document routing for individual content items?

and my response:

Think of Site Collections as separate gated communities.  Just because you can get into one doesn’t mean that you can get into others (you probably can’t).  To me, Site Collections are all about security.  The walls between the communities have broken glass on the top and guards roaming around.  Now, if you can get a guest pass, you’re all set…

At one of my clients, everyone got a separate Site Collection (over a thousand of them) and NOTHING could cross those boundaries.  Sort of what they wanted, but not really always.

which of course generated the clarifying question (my cool metaphor didn’t really work in the send to behavior of the USPS or anything):

So to be clear, “send to” and document routing would be inhibited by separate site collections?  Passing documents between site collections?

with the answer:

Correct.