One coach that I tend to hold in very high esteem is Charles Poliquin. That being said, here's what he says about seated goodmornings:
"I was introduced to this exercise 20 years ago by former Canadian national weightlifting coach Pierre Roy who, in turn, learned it from talking shop with his Polish colleagues.
Seated good mornings are a great movement for athletes who have yet to be exposed to intensive lower back work. In other words, I'm more likely to prescribe it to a beginner than to an Olympian. However, there are times when it's a proper choice for an elite athlete.
Every time I show this exercise to someone, I always make sure to warn him or her that it won't necessarily feel like much when they're doing them. I invariably get the following response:
"Coach, this doesn't work the hamstrings â?? I only feel it in the lower back."
I either slap them or tell that we'll "talk about it tomorrow." It never fails. I always get an apology the next day that goes something like:
"Oh, guru, how I can I have doubted you? I can't even sit on the john without screaming in pain. Please forgive me, I will never doubt your infinite training wisdom again."
Well, something like that. Since I use this exercise mainly in general preparatory phases, I tend to prescribe slower tempos (such as 3030) and higher repetitions (8-12). Therefore, I use it mainly for anatomical adaptations (building the size of the hamstrings, teaching recruitment, etc.).
I may even prescribe extremely slow tempo (up to ten seconds) for the concentric range. When I choose this route, I'm mainly concerned with creating intramuscular tension by forbidding the use of momentum. Of course, when I use such slow tempos, I rarely exceed six repetitions per set."