In reply to ourkid: It is partly about 'specialisation' - what does that 'coach' or 'instructor' do most of?
Their roles do overlap 'instructors' certainly all coach and even the self proclaimed 'coaches' often end up instructing basic gear placement. However, they will generally be best at whatever they do most of.
Also as you'd expect professional instructors and coaches all have strengths and weaknesses.
I have watched in awe as a famous 'boulderer' demonstrated the best gear placement and rope work I've ever seen on a classic sea-cliff traverse, far above what most professional instructors ever achieve, conversely I know of another famous climber making a complete hash of delivering a basic training session.
Probably the best two hours of instruction and coaching I've ever witnessed was delivered by an MIA who probably hasn't climbed harder than VS this year. Equally I know of other instructors who've ticked 'hard sport routes' who aren't up to guiding on multi-pitch routes over 6 grades easier.
The good news is that there aren't really any bad instructors/coaches, it's more a case of good and very good. If you are a beginner/improver, almost any instructor/coach will be able to help you. However, what this does mean is that almost every instructor/coach around therefore seems to get rave reviews.
Just because someone else had a 'good weekend' with 'Instructor A' doesn't mean they are best placed to offer what YOU want.
Anyone, thinking about using an instructor/coach, should speak to them in advance about what they want out of it. Everyone I know would be very upfront and honest about exactly what they are best placed to deliver.