Vet here. Its worth noting that the majority of vets are actually poorly trained in the benefits and drawbacks of neutering (the education hasn't caught up with current science). I work in this field so may be able to add a bit more actual science to the debate.
Recent research shows that neutering generally doubles the risk of cancer. For most dogs this isn't significant as the initial risk of cancer is tiny so doubling this risk still results in only a tiny risk. However in some breeds like rottweilers which have a higher risk of cancers like osteosarcoma (bone cancer) this risk can be significant. cebp.aacrjournals.org/content/11/11/1434.full
There is also evidence of increased risks in golden retrievers www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0055937
There are no anti-cancer benefits of neutering a male dog, to my knowledge (testicular cancer is vanishingly rare)
Prostatic cancer is more common in neutered male dogs www.ncbi.nlm.nih.gov/pubmed/12431819
Neutering a male dog will decrease the risk of prostatic hyperplasia but this is also a treatable condition
If a dog is fearful or fear-aggressive, neutering him can exacerbate this issue.
Neutering does decrease metabolic rate by about 20 - 25% and so he may gain weight. There's no evidence that food type has a significant impact link.springer.com/chapter/10.1007/978-3-642-36488-4_31
Neutering may be routine but it is still a painful, invasive surgery requiring a general anaesthetic and after care.
Dogs that are neutered are less likely to roam and seek bitches in heat, get hit by cars etc.
Responsible pet ownership is about ensuring your dog is under control and doesn't bother others - this is generally easier with a castrated dog and the majority of pet owners castrate for the social/owner benefits. This doesn't mean that there aren't any benefits to male dogs, but there certainly aren't as many as are commonly touted, and it is perfectly possible to be a responsible owner without castrating your male dog.
The benefits and risks are different for female dogs