Imaging a situation where I`m designing a new product and I need to
select an external ACàDC switch mode PSU with a very standard barrel
connection output. The product is a metal case, with chassis connected
to system wide 0v.

Examples below:
http://www.powerconversion.com/products/cat/57/Freestanding-ac-dc-pow...

I note some of these power supplies have the output 0V output
connected to the input Earth, but most do not…  (IE: the zero volt
outside of the DC Barrel connector is connected to the AC connector
Earth pin internally to the PSU). I see more and more laptop power
supplies using this earthed negative system...

What are the relative EMC advantages / disadvantages of having this 0V
à Ground connection?

I think earth loops may be a potential downside to this but the
earthed 0v seems like a great way of sinking some noise currents in
the system?

Many thanks to all,
anna

Common-mode suppression in a unit without the ground wire can be be
misleadingly assessed - various apps show compliance expected from
small converters with minimal filtering, simply because the output is
not grounded in testing. This is impractical in many real situations.

The ground wire in the supply cord allows EMC control over a wider
range of applications, as sheild currents can be returned locally and
safely. It's not easier, or cheaper, but it's more repeatable and
dependable.

RL