In the beginning, I would have never believed that I would be able to do three different venues at the same time. I currently participate in obedience, agility, and hunt tests - but the dogs can learn to differentiate, and it does work. What I have found is that by doing all three, it has made me a better handler.
Obedience at the highest level is very difficult and requires a bond between you and your dog that is far different that the field or agility. Let's face it....if a dog was left to do what he wanted to do, he might hunt ducks, perhaps jump over "obsticles" in the woods, but I don't think you would find many dogs doing attention heeling for fun. I like having to find a way to work with my dog to bring out that attention and it requires that I spend quality time each day trying to perfect things like perfect front and finish positions, as well as the ultimate heel position.
There are other issues too....try teaching utility articles to a dog that has been through FTP. In the beginning it is a challenge, until they figure out the game. I am not sure what everyone else uses, but for articles I use "find it" so that I don't ever confuse it with a go and grab command. As already described some of the dogs in utility confuse the jump for over piles. I find that hand position is critical here. All my casting signals start from mid body, my jump signal starts from my side. I am currently actually doing pile work in my utility set up and my dog has learned to differentiate between over casts and jump signals.
All of my other commands are pretty simple - the only other one that changes for hunt test work is "heel". For me, heads up, pay attention to me is heel. When I go to the hunt test line I do not have heads up attention, and nor would I expect it. I do expect that the dog stays with me, but there are birds in those fields....how could I even think my dog could heel with its head up eyes glued to mine when she thinks there is a bird. So my field word is "here", which still allows for good position, but relaxes the standard on the head position.
I was very happy to see that so many people actually participate in obedience. I was just at a show this weekend and there was a handler there that I know does at least master level hunt tests with her dog if not field trials. While her dog wasn't perfect by ring standards, he was motivated, happy, lightning fast, and one awesome retriever. What a joy to watch - and it is nice to see the smiles on the judges' faces for the extreme effort.
Happy Training!