They are fine for assistance - they are part of the exercises that you can choose to do. This is in the 531 book.
They are a tiny part of the Beginner Prep program. Along with lifting, running, calisthenics, jumps, throws and bodyweight work.
Are KB's a good option for hardgainers? They are a small part of any program, not THE program. Doesn't matter if you are a hardgainer or not. I can tell you that if your main goal is being a better athlete, the KB is low on the list of priorities. There is nothing wrong with them but there are 1000000 better ways to get stronger, faster, more agile, etc.
As you probably know, there needs to be efficiency for GPP for athletes. Do the minimum amount to get the maximum effect. Choose your weapons carefully.