Whilst the (re)introduction of the instructor test may well benefit the FIEs don't forget that it might also benefit the students.
As things stand after completing the initial FI test one can complete 100 hours instructing and get signed off as an FI and then instruct for the next 50 years with no check on ones competence ever again - just doze at the back of a seminar and complete the required number of hours.
In commercial operations ( and in the military ) pilots are retested on a regular basis, why should flying instructors be any different.
By the way I am not an FIE and have no financial interest in the matter - but I far prefer to revalidate my instructor rating by flight test as it gives me a chance to receive constructive critisism of what I am subjecting my students to.