OP, some elite 800m runners do incorporate short explosive sprints into training. For example, Nick Symmonds' group does this, as well as some other groups.
Some other groups seem to not include this sort of training.
At any rate, it doesn't have the same stimulus as weight training. Heavy weights are intended to increase maximum muscle contraction force. Short sprints and plyos are used to decrease the time it takes muscles to reach maximum contraction force.
Of course if you add short sprints into a program that doesn't have any weights, it will probably still help to increase your maximum contraction force, but not as much as heavy weights will do.
Of course, whether specifically targeting maximum muscle contraction in the middle distances is better than spending that energy on different training is definitely debatable. It is even debatable in the sprints (a small number of elite sprinters don't lift at all).
I'm inclined to include weights for more speed based 800m runners during the off seaso along with limited maximum velocity work, phasing out the weights and phasing in plyos as the season progresses. For more aerobically based 800m runners, I wouldn't include weights and would not emphasize speed quite as much, instead emphasizing speed endurance more with these athletes.