There are reasons to leave a dog intact until later, but they're not necessarily pertinent to every dog owner.
For extreme athletes, sport dogs, agility dogs, sled dogs, etc., testosterone strengthens and toughens the bones, muscles, and connective tissues, so it can be desireable if the dog is doing hard physical work. For the majority of pet owners who do not work their dogs, it's probably not an issue.
If the dog is neutered before physical maturity, his bones will grow quicker and longer, resulting in a slightly taller, leggier dog. The chest, neck and head on intact dogs continues to grow wider in maturity, resulting in a stockier, more "masculine" look. Neutering before maturity arrests these secondary male characterists. This may be a big deal to some, but not everyone cares about the size of their dog's head.
There are some cancer risks that are decreased with neutering, and others that seem to be increased with neutering. IMO, the risks sort of balance each other.
Neutering before maturity tends to take the "edge" off a dog, makes him a bit more compliant, and less likely to exhibit male behaviors like marking, roaming, challenging other males, etc. People whose dogs are doing bitework WANT that extra edge of aggression, so they leave their dogs intact. But neutered animals are generally easier for the average pet owner to handle and train.
The most important reason to neuter IMO is to prevent accidental litters. If you can keep your dog securely contained and do not allow him access to females in season, this may or may not be an issue.
My personal feeling is that neutering is a good idea for the majority of dog owners. It just makes things easier on everyone. If you are responsible and feel you are capable of handling an intact dog without EVER siring an unwanted litter, then I have no particular issues with leaving a dog intact.
Most pet owners neuter at 6-8 months, others wait 2 years, some don't neuter at all even if the dog will never be used for breeding. If I get a male pup, I will probably split the difference and wait as long as I can stand it before neutering; sooner if he's being a hormonal butthead, and later if he remains a gentleman, but I think I will try to wait at least a year.