What would I use a MATCH SIMPLE foreign key for?
My preference is to avoid nulls in foreign key columns. Make all foreign keys non-nullable. In fact I have foreign key = not null as a standard check in some static analysis scripts I use for validating database designs.

The FULL / SIMPLE behaviour is obscure enough that in my experience even experts don't always get it (I'm an expert and I have to think twice about it!) As ypercube points out, SIMPLE is effectively the default in Microsoft SQL Server but I suspect most people working with that product never even considered it. If one of your goals in database design is to make data comprehensible by and useful to the average user then foreign keys ought to be non-nullable.