In the latest version of the playtest rules, when you get it (at 5th level) you can do it twice. And it goes up every level from there.
Yeah, and I imagine that the fireball is going to be a little less impressive as a result. Still, the fewer uses of a thing you have, the more it hurts to get rid of it, which means that DotM for limited resources is less dull than DotM for at-will resources. The more limited they are, the more true that is (3e: probably not limited enough; 1e: possibly limited enough; 5e: maybe limited enough?). Exactly where that switch flips is probably a pretty subjective call, but two extremes of the continuum are, "DotM: on every attack I make" and "DotM: Once per day," so I figured that'd be useful for illustration.
EDIT: Let me put it another way. I have played 4e with a fighter who used Reaping Strike. No one keeled over from boredom.
'course not. But if you want to play Dueling Anecdotes, we can be here all night. Some groups/players have a much higher threshold for tedium or a much lower threshold for loss than others. I like a game with significant swing, because it creates more interesting emotional highs and lows, and DotM at-will is crazy dull while DotM limited-resource is much less so. One works, the other doesn't. It doesn't have much to do with favoring a particular character type (spellcaster vs. non).
FURTHER EDIT: Let me put it yet another way. How many GMs tell the players the number of hp remaining? If the players don't know, then they don't know whether or not autodamage will suffice to kill a target.
Also, sometime damage dealt on a hit is enough to guarantee a kill even if minimum damage is rolled. In my experience, that actually happens quite a bit. But the elimination of that element of the gambling process of D&D, on that occasion, doesn't make anyone fall asleep either.
Well, minions (and the 5e-equivalent) become player knowledge pretty fast, and rounding off the last 2 hp or so isn't a significant swing for most games, so getting rid of it is mostly just accounting.
But none of that really removes the dullness of DotM at-will. In 4e, Magic Missile and Reaping Strike are both pretty milquetoast abilities. Which isn't what I really look for in a game.
And which also isn't to say that such things should not exist, just that for the default assumptions of the basic newbie-friendly game, they could probably do better.