I originally wrote this post when there were fewer than 20 posts here on the topic, but my internet connection went down for 12 hours. Unfortunately I don't have the time to read through everything else right now since there seems to be a fair number of long posts in this thread however I will add my own thoughts on the matter and check back later.
Cheating is a personal preference. It's up to the player whether they cheat or not. The only difference between using a trainer and codes built into the game is about 5 minutes for a google search. Possibly a little more depending on the trainer that you are using. Looking at it like this I really don't think that it makes much difference to your average player (of the cheater kind). As others have already stated, having cheats built right into the game allows for easier debugging when patching or modding the game and as such IMO they should be left alone. Most if not all games have some kind of Debug cheats in them, so I see no reason to change it.
If you have a problem with someone cheating while playing your mod, just step back and think about it for a moment. What difference does their cheating make to you? Unless the person cheating is doing playtesting for you and is giving bad feedback about the difficulty because of their cheat then how does it affect you? If that was the case then you could justifiably be angry.
As it stands you seem to be complaining about other people cheating and thereby ruining the game for themselves because it will no longer provide a challenge to them. My question to you is: Why do you care how others play the game? If others want to cheat then more power to them. Whether or not the cheats are built in or not will not stop a cheater. It is a fact of gaming that people WILL cheat. There is nothing you can do about it.
Your objection was based on others ruining the game with cheats. However have you considered that for some people, cheating can be fun? Have you never felt the joy of covering every square inch of the map with zerglings in Starcraft? Or becoming wealthier than Bill Gates in Simcity? For some people these activities can be fun and in some cases can only be accomplished through cheats. Some people enjoy trying to break games. I personally used to love cheating. I remember playing the Command and Conquer: Red Alert and modifying certain units to have a cost of 1, 5000 health, maximum speed and a tesla coil as their primary weapon. Adding all these to spies was the most fun as certain allied mission had you control a single spy and required you to avoid guard dogs that could sniff you out. After being eaten alive 40 times, roasting those little dogs with 50,000 volts was extremely satisfyling.
With all that said, I wish to make it clear that this view is only for cheating while in single player. Cheating in multiplayer is completely different and while it may not ruin your fun to have auto-aim and wall hacks, it does ruin the fun of others when the get hit with a head shot while underground by some 13 year old from across the map. Cheating in multiplayer is a terrible thing, however when it comes to singleplayer, let people play how they want.
