I don't think mike is saying its bad for you, you are just confusing the purpose of each. Power endurance work is good for an 800m runner, but not if you are trying to increase max velocity (get faster). Try boudning 200m on the track, tell me what distance that feels like racing. Are 400m repeats bad for the 800m runner? No. They aren't speedwork as disscussion on this board goes, in distance coach/runner cirlces they truly shouldn't even be considered speedwork,but pacing, because 400m+ intervals are about pace, they fall into power-endurance category as well, anything working about 15s to 100s (120m to 600m) is special endurance work. In fact special endurance work to me is really power endurance which is a product of speed-endurance and strength-endurance, the less strength-endurance usually means the less power endurance or special endurance, meaning strength work is critical and should be considered before power endurance work to maximize strength and strength endurance. If you increase your maximum strength you'll find you can increase your submaximal work volumes doing high volume medium intensity plyos for instance. Unless you are using the plyos as part of a general strength routine or a circuit, you should not think about making them event specific work until mid to late season to give some variation to those 200,300,400,500 or 600 meter repeat days. One thing I do is split my plyo sessions on strength day in the weight room we do speed/quickness drills (quick foot contacts med-low intensity at med-high volumes), on the power day we do height/distance (med-high intensity @ lower volumes) drills, the key is rest between sets and reps. If we skip for 20-30m we rest about 3-4 minutes before the next interval of skipping, if we jump to a height we rest 30-60s. Sometimes I shorten the rest to test them for adaptation. I have to agree with Mike plyos and wieghts go hand in hand and I work them the same day, I like to complex them to (follow a lift with a plyo drill or vice versa)
You also need to remember that the 800m race is the ultimate paradox. It requires significant aerobic metabolism and power capabilities, borderline on being great in 1 mile-5K races and it requires sufficient speed that your speed needs to be good enough to being borderline great in 200-400m at the level you want to compete. I am being kind of loose with the use of 200m and 5k distances, but at the lower levels of competition youth and high school there is not much difference between great 800m runners being good at both those distances. Training the extreme ends of both speed and endurance is critical, ie max V and LSD.