I was tempted to go for that a few times, but never went along with it. If I'm able to express knowledge to some one in a way that they can comprehend and use in the proper way then I don't see a need a certificate from a course.
On the other hand, a certificate from an instructor's course will put those students at ease who look only for teachers who are "qualified" by a grand master. As to the quality of the course, I think it would depend on the schools advocating them.
I've seen some courses where you pay a fee, show up for like a week or two of classes, then you have a certificate. Some cheesy ones do the week course without previous experience (but perhaps costing more)! Other instructor courses I feel are more reasonable, taking up to a year with minimum durations and tests, before one is qualified to teach that system... basically an accelerated course focusing on the most important points.
Before I began teaching, I thought taking an instructor's course would be a good idea, helpful to be established, but looking at the price of the course and the material studied, I felt it was way overpriced.
IMO, experience makes a good teacher, not a week long course. so if your sifu was good, and taught you well, your understanding is good, and you can express what you learned clearly so the students can learn, then there's no point for a certificate.
One that might be useful though, rather than certifying for a whole system in a short time are those specific courses, like taking a week long course to learn a certain form, or certain set of techniques, then being qualified to pass on that knowledge...