Basically I'm asking has the Minimum Client Version check been put in for the benefit of the DF developer or are there reasons why the DF run time requires a minimum client version
Following is from the help:
Use of an older client with a newer SQL Server version can in some situations cause problems. One known issue is the use of the SQL Server ‘date’ type. This type was introduced in SQL Server 2008 and is not recognized by older clients. An older client will return SQL dates as string (DF_ASCII), which can cause incorrect date values in the database.

On most Windows systems the “SQL Server (SQL Server 2000 client)” is installed as part of Windows. If no newer client is explicitly installed, that is the client that will be used. This may lead to a situation where a too old client is used.
We have had quite some support calls where developers deployed an application and they did not realize they were using a very old SQL Server client. At first sight everything appeared to be working, but after a while errors and even incorrect data could occur.

So yes, if you get this minimum client version message you really should install the correct client.