EPTHallucination
If it did not cost so much and did not require costly research, it might see some use, even despite competing with storm. As of now, storm is always better, sadly. Only on very very rare occasions hallucinations are used to make sure that arbiter or shuttles can get into fortified terran base with turrets under their cover.
Mind control
Has only single very niche use: steal SCV when protoss get maxed, to put terran on death clock timer. Does not work on pro level, though.
In theory mind control could also be used to counter shuttle drops in PvP, but games mostly end before someone can get a DA and research spell. Also it hard counters carriers and BCs, but both are not built vs protoss...
Feedback
Sometimes used on defilers and high templars. Very cost efficient counter, though not popular on pro level, and dark archons almost never get built just for that purpose.
Disruption web
I guess by design it was supposed to be used against terran, but since it costs so much, does not last long, and requires to build otherwise useless in PvT unit... basically no one casts them.
The only competitive use for d-web is corsair-reaver strategy, since you already have fleet of corsairs anyway, and do not want to lose expensive reavers to hydralisks too quickly.
Restoration
Another almost useless spell. It does not cost much energy, but requires research and extra micro. And there are not many things worth dispelling anyway, this is not Warcraft. The only competitive use is to save BCs from their common counter plague, but even then most pros simply let battlecruisers die and focus on other things.
Optical flare
What a joke of spell. Has no real use...
Parasite
Very rarely seen in very late ZvP, if zerg needs to see where HTs are on super defended base.
In theory can be good vs mech, giving vision of Terran mech army, and to put on a SV: vessel already costs more than a queen, so for 75 energy you've either forced Terran to throw away a unit that costs more than queen or give yourself a vision advantage.
Ensnare
Good counter to mass wraiths, if terran player`s micro is too good and wraiths are too annoying. Ensnared wraiths cannot cloak and get killed by mutas very easily.
Can be used on group of bio units, too, but usually is not worth extra micro, resources and energy demands.
Also rarely seen in muta wars. Spending resources on queen and ensnare instead of more mutalisks is risky, unless you already have huge army, but it pays off later: ensnaring stack of mutalisks basically guarantees a victory.
Infest Command Center
Infested terrans are pointless units in ZvT, however finishing off CC quickly and getting flying buildng to scout with is a good move. Of course you will not build queen just for that purpose, but even if pros already have queens built, they usually do not bother with infestation for some reason. Maybe because it requires extra apm.
Lockdown
Another very weak spell. In theory it is a perfect counter to BCs and carriers, but cost, research time, vulnerability and most importantly micro demands result in everyone not bothering to build ghosts.
Nuke
Used in rare nuke rush strategy. Also rarelly used to nuke tank lines in late TvT. Still more useful than lockdown, though.
EMP
More popular than other spells above, since it is a default counter to arbiters trying to sneak into terran base to recall. On rare occasion vessels emp HTs, too, if protoss is not careful and groups them together before assault.
Can find a use in late TvT in battlecruisers wars, since yamato is so powerful spell.
In theory lucky emp hit could counter queens, which are popular counter to terran mech.
P.S. Spawn broodlings and maelstrom in the past were almost never used spells as well, but now they are popular powerful counter to mech / mutalisks sniping HTs. Which shows that pro players are generally very conservative, since so many years have passed...
