Ah loot cheats.
It could and probably will happen, to an extent. However, it would be an odd way of playing, because there is no benefit to cheating. It would be like unlocking 'God Mode' in a game and having one hit insta-kills... you may get through the content in record time but the play experience is going to be pretty dull.
By fighting opponents and learning from your mistakes, you realise your strengths and weaknesses. So when you do get those uber-rewards for defeating that tough enemy, you know what you need to take to improve your hero's performance. Someone who is not bothering with the combat would have no clue if that 'sword of ultimate smiting' is a better option than the 'cloak of over-powering stench', for instance. (Note: those names were made up... *cough*)
If your hero dies in DQ you don't start over. That would be dumb. There is a different system in place. You are not penalised for dieing; you are encouraged to learn from your mistakes and try out different strategies. People will want to do the combat because that is where they will show their skill and knowledge...
And hey, with the hero vs. hero rules, someone who cheated through the content would be shredded to pieces by a talented player anyway. It would just be total 'pwnage' ;-)