To each his own. Many riders enjoy arena work; others prefer trails. Personally, I love when I have access to a nice arena with good footing to school in, but that's just not always possible, so I do most of my conditioning and training out on trail and in fields. I have access to a fair amount of land from my barn, and the metroparks are very close by and offer miles of trails. I'm fortunate in that none of my horses ever had major issues with trail riding; Excel was probably the worst and would awkwardly jig along sideways when I first brought him home from the track, but after a few months he settled down nicely and is now the bravest cross country horse I've ridden.
Then again, I'm the type of rider that gets bored just walking on trails, but I completely respect riders that enjoy that. There's nothing wrong with trail riding, and there's nothing wrong with not trail riding, just whatever best suits the horse, rider and environment.