As someone who likes to take a couple of seconds to look at their creation every few changes, I go in and out of build mode quite often, and the warning sound starts to get mildly annoying after a while. First, it confuses me because I think I'm under attack. Second, it startles me because I keep forgetting that it'll happen, and third - it doesn't really tell me anything I don't know. I'm well aware auto-saving is disabled. I know because I turned it off myself. Pestering me that it's disabled isn't going to make me enable it again.
EDIT: Bonus: I always listen to music in the background whenever I play, so the sound really interrupts that nice flow of whatever I'm listening to. That actually annoys me more than the abovementioned issues. Turning off the in-game sounds is not a solution because I need to hear when something important is actually happening.