Nature did not creat the domestic dog, people did. Being in heat, or smelling a female in heat can be very stressful on the dog 'nature' is telling the dog to mate, and not being able to can take a tole on the body. 'That' is the only difference I saw when my male was neutered, he no longer went through being clinging and whinny and restless everytime my neighbor's dogs were in heat, it was a small thing that at the time I didn't think much of, until he was neutered! Then I noticed! He seemed so much more relaxed and happy! Nothing about his personality or drive changed. He's 10 years old now, which is good for a dane! Neutering hasn't hurt, or slowed him down any! My female was spayed early, she's currently 7 1/2 and healthy as a horse (both her parents died at 8, I think Havoc's dad did to), she's slowed down some (thankfully, my family would say, they don't miss the white tornado), she's always been more puppyish and very active, she's my trail buddy! My chinese crested was spayed later, and honestly other than not missing the horrible heats, there was pretty much no change in her at all, she's the same as she's always been. I admit spaying her was as much for me as it was for her, I couldn't stand the heats and had desided not to breed her, so why deal w/ it. And I think it's better for her to, during the time she was in heat all she wanted to was to get out and hook up w/ the doxie standing outside the door, no desire to eat, she was restless, whinny and annoying, I couldn't even take her outside because that other dog was there (thankfully he was the only one, maybe no one else could break into my yard).
For a long time I didn't really see the need to neutering, spaying yes! Don't want any babies, but neutering not so much, I've changed I guess, I just think they live happier, less stressed lives. It may be ok for a male that is never w/in smelling distance of a female in heat, but a female can not excape it. Now I do think I prefer to wait on a male til he's over a year old, w/ a female I prefer to do it before she comes in heat for the first time.
That said I think it's a choice each owner should make on their own and shouldn't be pushed on them, as long as they are responsible for their pet.