If you manually control them they will continue to fire at the target until its killed. If you let them automatically attack they will attack a target until its engines are gone and switch targets.
I believe manually control makes shots being wasted. Since they are programmed to not waste shots when automatically shooting.
Of course I have seen them attacking engine dead ships, this is mainly because their are no more targets left to have their engines destroyed so they act like regular snipers now. Then new ships enter the system but the spider turrets are still attacking their target until its killed, so that's the only time I manually control to target 1 ship with the lowest health so they can get back to their job.
I have never used a C.node for spider turrets but I would assume they would act normally when targeting engines. That would be if the targets engines health are very low only the amount of spider turrets needed to take them out will attack, the rest should target the next target. Allowing no shots being wasted.
Unless its not programmed to take in account how much the engine health currently is, then all spider turrets will attack that one target regardless if it needs just one more shot to have its engines taken out. Making shots being wasted.
All I have to say is zoom in and watch, testing is required to see which hypothesis can happen.