I don't know why I'm throwing myself amidst this argument but...
Because you want to test otu the asbestos firesuit you just bought on EBay?

1. Modifying the game (externally) to give yourself an advantage (no matter how slim) is cheating.
By your own self-conflicting logic, modifying the autogen.cfg file is cheating. That is, by the very definition, a modification done to the game files externally. If you actually mean modifying the executable or .pk3's is cheating, again, custom HUD's do not modify either of those. They re-arrange existing graphics, nothing more. Custom HUD's cannot, for example, force-enable the human radar effect, despite what you may have heard to the contrary.
Think of it this way: Let's pretend...
Except the crosshair
isn't slightly up and left of the center of the screen. This argument is what is known as a 'moving the goalpost' falacy.
1b. Everyone says that in Q3 you were allowed to do this and that, blah, blah, blah. This is NOT Q3. As a side note (slightly off topic), some people are using GUI mods to add in permanent radars for humans.
No, modifying the client-side
executable to force the 'helmet radar' on all the time is the same amount of effort, and the same type of cheat, as an aimbot. Modifying the HUD files to add/remove a crosshair, or any other visual decoration, is an ability the Tremulous developers specifically
added to the game. They chose to make the HUD able to be modified and customized. They had to make extensive modifications to the stock Q3 codebase to do this, so
it took extensive effort on the Tremulous developers part to allow us to customize the HUD.2. Binds are not cheating. These can be added in game and are used by the UI to set preferences, such as WASD for movement. Most people modify binds externally however this is merely a one-shot time saver as opposed to a cheat because you can enter the exact same information through the console in-game.
So setting r_picmip 7, r_gamma 5, r_mapoverbrightbits 4, r_intensity 0.5, r_vertexlighting, and turning Tremulous into Stun Runner visually so everything stands out like a sore thumb, that's okay with you because it can all be done from the console? You can add map .pk3's, and other files to the Tremulous directory such as "human" and "alien" binds I can execute easier, but somehow typing in "ui_hudFiles ui/custom.cfg" is cheating when anything else I can type into the console is fine? You can't have it both ways.
3. I think everyone knows that aimbots are cheats. Just to be clear, though, aimbots are cheats.
And so are all client-side
hacks as well. Modifying files specifically coded to allow the user to modify though, how is that cheating? Really, the Tremulous developers had to do extensive work to allow custom HUDs. They chose to allow them when they re-used the HUD system from Team Arena and updated it to support the Tremulous-specific attributes and items.