The argument here is simple really. As the sinister, evil-genius baddie who has gone to all the trouble to hide my secrets, and my loot, six or more levels below the earth and guarded them with all sorts of fell beasties, foul magics and traps the reason should be crystal clear. I don't want ANYONE coming near my stuff. Especially a band of those pesky, meddling, and potentially quite dangerous, adventurers!
Now sure, there are other purposes to set traps. Maybe I want to maim someone, or slow them up so they'll be easier for my minions to take care of. But why main or cripple or slow someone down when I can just kill them with the trap itself? Maybe it's to keep my somewhat rock-brained minions away from places they shouldn't be meddling, but a key and lock does that job cheaper and costs me less minions. Or perhaps mark them with permanent die, so I know who opened my spellbook when they shouldn't have. There could be lots of reasons, sure.
But the most straightforward reason to set lethal traps is to kill people I think are a pain in the asp. Death traps make all the sense in the world. Of course, these traps need an off mechanism of some sort, and maybe even a reminder or sign to help me and my minions remember where they are. Which makes the whole process for adventurers a lot more fun and manageable.
Now, I'm not saying every trap should be lethal. As outlined above, there are other reasons to have traps. But don't shy away from the good old save or die trap just because players think they suck. Provide a different avoidance mechanism the players can figure out, and go for it. It only makes sense.