So I read most of all the posts(It got a little repetitive. Sorry.) and there are things I agree and disagree with/on. I'd like to share my views. If that is alright with you all.
First of all, I am not limited. I ride or have ridden most disciplines in most types of riding and on most horses. True to my user name ( ;) ) I love trail riding. However, I also ride and train horses for show and know the importance of an arena.
Of course, your initial training will take place in an arena for the safety and less distraction factor but even when I am with a youngster, I will take them out and literally walk next to them on a trail. Why? Exposure training of course. Often, you will find that a horse that has been exposed to the various "scary" things that appear on the trail, will be more confident in the show ring.
Ok, I know, I know. I sound stupid right? I took the pony I was working with over the past 1 and a half years to his first outside show about 2-3 months after he came to me. In that time there was all the lunging and riding in the ring to check out the quirks and vices but soon after that we were hitting the trail. He had been under saddle for a month before we got him( He was 13 years at the time of backing and had never been exposed to anything. No arenas or trails, etc...) and it was clear that he was rushed but the trail riding helped him to calm down and listen to a rider. The show ring at the outside show was in a grass arena(although warm-up was sand) and there was tinsel all over all the jumps. Now, if we hadn't been exposed to trail riding... he would have done something funky when he saw the tinsel.
I think it must be in balance. All horses and humans are different, I agree, but horses also love to go outside, smell the fresh air and gallop around and that is with or without a rider. Remember, horses feel you too. So if YOU don't like going out on trails or if YOU do not like going into an arena, you are already putting your horses off it. (Please, the capitals are not an attack on anyone. It is just simply to put a message across.)