-If P has like 20 carriers your ghost won't help much
-If you make ghost you don't want your enemy to see them before they are ready to shoot (mostly it will be a surprise during a big fight, caching offguard, flanking, even hidden in dropship possible)
-Goal is mostly to hardly reduce or eliminate the first carrier stack so you don't have to deal with a shitload of carriers later on
*Ghosts are excellent vs arbiters
*vs BC it's also difficult/situational. BCs are slow but since you will encounter BC+Tanks it's often hard to catch them. Also in TvT everything will be scanned continous and your opponent will be prepared for sure.