AD Sites and Services

We are running an Enterprise environemnt with 3 major 2003 AD domains under a
root domain spread out acroos many subnets but all with-in the same general
vicinity and with fast reliable connections between the DC's-DC's and DC's -
Clients.

The original administrators never set up any subnets beyond the default
settings
in "AD sites and Services" and the default site. Whats are the potential
risks to not having any subnetts defined? What type of issues might we come
acoss ?
Currently we are adding another domain in another location, but with a fast
reliable link back to the home network. We are up in the air about creating a
seperate site for them. Some admins feel the extra replication
traffic/authentication is manageable.

I know that MS Best practice is to define all subnetts and sites, and I have
found a large number of articles related to what site/ipsubnetts do and how
to properly configure them. What I havent found are any articles related to
the pitfalls of not having anything defined.