Just to be able to copy a selected effect/instrument with settings you have made earlier.
Edit: Moved to issue tracker: http://bugs.openmpt.org/view.php?id=35
Are you after plugins, or just the instruments tab?
As cloning mod instruments is already possible, I assume he means VST(i)s by "effects/instruments".
vst effects/instruments. Like with the rigth mouse button

(http://bildr.no/view/523505)
I've requested this in the past too (years ago even)
Rewbs suggested that until this feature would be implemented, you could save the preset to disk, open a new instance, and load the preset.
Or just open two instances and changing the settings from one to another like I'm doing today.
So, it's just a timesaver thingy.
Just bumping this one here.
Bumping bugs helps sometimes, but not bumping feature requests. Actually, this is has been requested years ago already, so I wouldn't wonder if it take another x years to implement.