It is generally accepted that abilities which reward micro are desirable. These abilities can be repeated very quickly, and each action increases the effectiveness of the ability. Does that mean abilities that cannot be repeated are undesirable? What about abilities that have a high effectiveness within only a single action? Let's find out.
Let's use the baneling as an example. It's ability can only be used once. When a baneling blows up, the unit dies and you can no longer use it. In order to compliment this low repeatability, its individual effectiveness is very high. One baneling can potentially blow up an large number of small light units. A lot of unit movement micro can be applied to the baneling, but none of the actions increases its effectiveness until the critical moment when it blows up.
In isolation, the baneling ability seems very undesirable. You only have one chance to use the ability. Since this ability is not repeatable, it's effectiveness is very hit or miss and is not proportionate to amount of micro applied to it.
However, since the unit is cheap enough that a player can make many, many banelings, the ability becomes more repeatable and a flood of banelings becomes increasingly effective based on the player's ability to micro its unit movements. The AOE damage can be maximized by splitting the banelings, burrowing them or by running them past armoured units to reach light units.
The widow mine is a very similar unit. It has a single ability that has quite a long cooldown. On its own, the ability is not repeatable and its effectiveness is hit or miss. However, since the unit is cheap, you can make a lot of them. Burrowing and unburrowing, splitting, and unit targeting are all actions of micro that can be applied to a large number of widow mines in order to increase their effectiveness.
In Legacy of the Void, the Protoss race is given a very similar unit, the Disruptor. This unit has a single ability with a long cooldown and deals massive AOE damage. It is unique from a baneling because it is invulnerable during its ability and does not die at the end. However, this unit is expensive and cannot be massed. As a result, unit movement micro cannot improve its repeatability or increase its effectiveness as much. The unit suffers from the lack of these micro aspects and its ability becomes more hit or miss.
With that said, does this make the Disruptor a bad unit? Maybe. Maybe not. There are other units with similar abilities such as psionic storm, fungal growth and emp. The units associated with these spells are potentially more hit or miss and benefit even less from unit movement micro than the Disruptor. What's the difference? That's for next time.
Vol II: Love Hate Force Fields
http://www.teamliquid.net/forum/starcraft-2/488655-breaking-bad-micro-vol-ii-love-hate-force-fields
Vol I: General Micro
http://www.teamliquid.net/forum/starcraft-2/488425-breaking-bad-micro-vol-1-general-micro