It can be a bad idea to over-walk very young dogs. I don't walk mine for long anymore- he had very bad untreated breaks as a pub (rescue) and is arthritic now so he toddles around the village and no further.
For a healthy, adult dog - I can't see how it can be bad. Certainly the health implications of no exercise would be much worse than any reason I can think of.
We have a one yr old lab and on the vet's (and fellow lab owners) advice we have followed the 5 minute rule. So a 6 month old would need 6X5=30 minutes per day and an 8 month old would need 8X5=40 minutes per day. This is to try and minimise hip problems(which labs are prone to) in later life. I don't know if it really makes a difference, but we were not happy to risk it. We were also told to avoid letting her use stairs and to not let her jump in and out of the car.
She has a walk everyday and has the other half as off lead time over the forest. So far she's in great shape.
I have a doodle and my vet said not to over walk him as it can bad for him on the Lab side for the hips. He doesn't have hip dysplasia but they thought it's a good idea not to go mad with the walkies!!
Odd thing for a vet to say on the one hand, PC, given that both breeds, labs and poodles, are traditionally working dogs.
On the other hand there is so much unscrupulous breeding of all dogs, especially these doodle mixes, that the vet is probably erring on the side of caution. So many of them do not come from hip scored parents and so may well end up with problems in years to come.
I would add that i havent taken the vet's advice completely. My lab was diagnosed with severe hip dysplasia when i got him, he was 2 years old. I have always taken him for gentle on the lead walks, he is now 7 and he isnt in any pain.
I too think that vets can be a little too cautious sometimes.