I think that some dogs will just let the vet or the groomer do things that the owner is not able to do. I have a friend who had an enormous Golden Retriever, and that dog would do about anything for Jill, she loved her so much. I think that Jill was just afraid thst she would clip the nails too close if she clipped Maggie's toenails herself. Not only that, Maggie was abig dog and her toenails were large and tough, so the vet probably had better clippers to do her toes with.
Now, she just has Ruthie, he little Pom, and I think that she iss fine to have her toenails clipped, and I have helped Jill clip them , too. But most of the time, Jill takes Ruthiss to the vet to be clipped, too.
I think it is just one of those thing where you have to make the best decision for yourself and your pet.