Dreams are very delicate, even the slightest change in a dream can completely change its meaning. Seeing Any of the Prophets or the ahlulbait in the dream is regarded as good but this is not always the case.
Lets take 2 dreams for example, in one dream the prophet is seen standing on a dry, arid land and in the other he is seen standing is someone's court yard which is also dry and arid. In the first instance the interpretation would be that the dry land will become green and verdant, while in the second instance the interpretation would be that fire and destruction will visit that place.
Rasuallah Has said: "Whoever sees me or my ahlulbait in his dream has truely seen us. For the devil is unable to assume our form."
So any one who sees such individuals in his dream has inface been blessed with their presence but the other details of the dream would point out as to what the Imam or prophet was sent to inform the dreamer of. One must remember that some times sinners are also blessed with the presence of such individuals so that they would take a lesson from it and quit their sinful acts.
Every dream has its individual meaning, Seeing imam mehdi and his army could mean a number of things, the intepretation would vary from person to person, the details would vary hense altering the interpretation.