If enough -1/-1 counters are put on Dockworker Drone at the same time to make its toughness 0 or less, its second ability will see all of the +1/+1 counters it had when it died as well as the -1/-1 counters it had, and an equal number of each of those types of counters (plus any other applicable counters) will be put onto the target creature
Other types of mana will continue to empty from your mana pool as each step and phase ends
In Magic, you lose the game if you try to draw from an empty library, and so the other, more common kind of Mill deck tries to get somebody to dump their deck and 'Mill out'
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Now, your opponent will want to commit significant resources to keeping his loyalty from getting that high, and that will also give you some value
There are also a lot of bad commanders and a quick look at gatherer can find generals that are literally unplayable (Haakon, Stromgald Scourge), ones that add nothing to your deck (Mishra, Artificer Prodigy) or just ones that are confusing and useless (General Jarkeld)