Guppies wont eat an adult ghost shrimp, nor an adolescent one... They'll eat baby ones though.
And the benefit they pose is they eat detritus and leftover food chunks, and they also groom plants occasionally. Not to mention they are good lookin shrimp.
And the bad thing about ghost shrimp is that they'll eat anything they can catch as far as fry is concerned... They also will eat pond snails, though not as much as I'd like, lol.
They do great in a planted tank, however plan on buying maybe 2 to 3 times as many as you want to end up with, for some reason they don't do all too well when you first get them home from the store. Also your fish will bother them into hiding most of the time.
But they don't harm anything.
Most people prefer amanos for their planted tanks because they eat so much algae.