When someone asks me for a deathmatch, I might spawn kill them until they quit or manage to escape, assuming I'm not busy with something else. That's enough of a deathmatch.
I accept 1v1 DMs if I was already planning on fighting them for a while AND I'm relatively sure no-one else will interfere.
The money sucks (at +$100 for 5 kills you'd be better off attacking 5 pedestrians) but it means you get RP from kills when you wouldn't otherwise and the kills count for your DM kill total and Dominating plat awards. (You can get about 400-700 RP total, which is better than the 0 you get outside the 1v1 but still it's kinda marginal).
1v1 DMs are only the slightest bit worth doing if you're confident you can very quickly and that nobody else will come along and kill you. It's just a terrible system all round really.