Hikoushi: Spot on 100%. I'v spent years as airliner trainer & checker, but believe a check should also be a learning experience. F/O's who spent 1 year as SFI said how much it enhanced their command course and opened their eyes. For myself, being an aerobatic SEP student again contributed hugely to reflect on my own instructional technique. Indeed being a student in anything can make you a better instructor in another field.
Being an instructor of a spectrum of students can make you a better operator. It was also wonderful to do learn so much doing checks on line crews. There are some good ideas out there.
The comments about CRM techniques and social mixing/tolerance are also spot on. Not all things can be learnt/taught on an condensed MPL course.