mad_jock asks;
In respect to practising IFR for an IR valid pilot.
As long as they are flying under Instrument Rules what does it matter if they are under a hood or in cloud?
A lot of people forget this fundamental.
When a flight is in VMC, the flight rules under which the flight is being conducted have no effect on the crew's legal duty to maintain a look-out and to
see and avoid.
If the pilot of an aircraft operating in VMC elects to obscure his vision, this requirement doesn't somehow disappear. This is the underlying reason for a safety pilot in the FAA rules. It also explains amongst other reasons, why the safety pilot does not need to hold an instrument rating.
In the UK, the strong line drawn between controlled and uncontrolled airspace protects people from one another to some extent. In most other parts of the world, where VFR and IFR traffic can come into closer proximity, the importance of this see-and-avoid principle is more obvious.